Genetically modified reduced-browning fruit-producing plant and produced fruit thereof, and method of obtaining such

ABSTRACT

A genetically modified fruit-producing plant, said plant having sufficiently reduced total Polyphenol Oxidase (PPO) activity relative to a wild type of said plant to reduce browning in the fruit of said plant relative to said wild type, wherein the reduced total PPO activity results from a reduction in activity of at least two PPO isoenzymes in said plant relative to said wild type, or a cell, seed, seedling, part, tissue, cell, fruit or progeny of said plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority from U.S. Provisional Patent Application No. 61/031,821, filed on Feb. 27, 2008.

FIELD OF THE INVENTION

This invention relates to a genetically modified fruit-producing plant, plant cell, seed, seedling, progeny thereof, or produced fruit thereof, which plant produces a reduced-browning fruit. This invention further relates to a method of obtaining such.

BACKGROUND OF THE INVENTION Browning of Fruit

Browning of apples and other fruit from damage, such as cuts, bruises, slicing, juicing, cell death, or any other form of damage that disrupts cell membranes, is believed to be caused by the enzymatic reaction catalyzed by Polyphenol Oxidase (PPO). The brown pigment is a polymer formed from the non-enzymatic condensation of quinones, with lesser amounts of amino acids and proteins, into lignin-like compounds. The quinones are synthesized from di-phenols in the enzymatic reaction catalyzed by PPO (Whitaker and Lee, 1995). Most PPOs also have monophenolase activity and convert monophenols to di-phenols (Mar-Sojo et al., 1998). The cause of browning has been understood for some time, yet the solution to reducing browning remains an on-going problem for industry.

Browning reduces quality by causing detrimental flavor and nutritional changes (Eskin, 1990). With the explosive growth of the fresh-cut produce sector, lost opportunities have become more evident given that, for instance, browning limits the use of apple and other fruits in commercial fresh cut produce products. It is thus a significant problem limiting the widespread introduction of fresh cut produce products such as prepared apple slices.

Browning is also a major consideration in the manufacture of juice, and brown bruises are a significant cause of reduced grade for growers and of lost value for institutional processors (restaurants, hospitals, etc.) and retailers, who have to accept these losses or try to minimize them through the implementation of improved handling practices.

Approaches to Control Browning

Various approaches to control vegetable and fruit browning have been described and resulted in mixed success, due to a variety of reasons, including cost and amount of handling. For a general review of strategies for reducing fruit browning, see e.g.: Friedman (1991), Iyengar and McEvily, (1992), Whitaker and Lee (1995), McEvily et al. (1992), Sapers (1993), Weemaes (1998), Martinez and Whitaker (1995), and Brushett (2006).

U.S. Pat. No. 5,939,117 (Cheng et al.) and U.S. Pat. No. 5,925,395 (Cheng) describes an anti-browning/anti-oxidant dip treatment. Fresh-cut apple slices which have been treated with an anti-browning/anti-oxidant dip, are described as having reduced browning. However, the off-flavoring and high cost of the anti-browning/anti-oxidant dip solution has limited their commercial success. Furthermore, anti-oxidant dip solutions do not deal effectively with secondary browning that results from the cutting knife and skin deformation prior to cutting and other secondary browning reactions, which lead to a thin brown line under the skin on the apple slice and other market detracting attributes.

Other approaches to control browning have been described, including but not limited to, cultivation in low oxygen atmosphere and low temperature (Heimdal et al., 1995); treatment with calcium ascorbate, glutathione, cysteine and citrate (Jiang et al., 1998); treatment with sulfites and sub optimal pH and high-pressure carbon dioxide (Chen et al., 1992); treatment of fresh cut apple slices with natural products (Buta et al., 1999); and a treatment with a 10% solution of honey (Osmianski and Lee, 1990).

Murata et al. (2000 and 2001) report that by suppression of a single PPO gene homologous to the apple PPO gene APO5 they obtained apple shoots and callus having reduced PPO activity which exhibit low browning potential in vitro. However, these references do not disclose a reduced browning fruit-producing plant or reduced-browning apple nor whether suppression of a single PPO gene homologous to APO5 would be sufficient to obtain such a reduced-browning fruit-producing plant or reduced-browning apple.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a genetically modified fruit-producing plant, said plant having sufficiently reduced total Polyphenol Oxidase (PPO) activity relative to a wild type of said plant to reduce browning in the fruit of said plant relative to said wild type, wherein the reduced total PPO activity results from a reduction in activity of at least two PPO isoenzymes in said plant relative to said wild type, or a cell, seed, seedling, part, tissue, cell, fruit or progeny of said plant.

In another aspect, the invention relates to a method for producing a genetically modified fruit-producing plant, said plant having sufficiently reduced total Polyphenol Oxidase (PPO) activity relative to a wild type of said plant to reduce browning in the fruit of said plant relative to said wild type, said method comprising reducing the activity of at least two PPO isoenzymes in said plant relative to said wild type.

In another aspect, the invention relates to a nucleic acid construct comprising: a promoter; a first nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 19; a second nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 21; a third nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 23; and a fourth nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule set forth in SEQ ID NO: 24 and encoding a polypeptide of SEQ ID NO: 25; wherein the first, second, third and fourth nucleic acid molecules are operably linked to said promoter in sense orientation.

In another aspect, the invention relates to a nucleic acid construct encoding an mRNA capable of forming a stem loop structure, the nucleic acid construct comprising, from 5′ to 3′: a promoter, a first set of nucleic acid sequences, a spacer, and a second set of nucleic acid sequences, said first set of nucleic acid sequences comprising: a first nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 19; a second nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 21; a third nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 23; and a fourth nucleic acid sequence comprising at least 200 contiguous nucleotides of a nucleic acid molecule encoding a polypeptide of SEQ ID NO: 25; wherein the first, second, third and fourth nucleic acid sequences are operably linked to said promoter in sense orientation; said second set of nucleic acid sequences comprising: said first, second, third and fourth nucleic acid sequences operably linked to said promoter in anti-sense orientation; wherein, the first and second sets of nucleic acid molecules are separated by the spacer.

In another aspect, the invention relates to a genetically modified plant cell transformed with the nucleic acid construct as described above.

In another aspect, the invention relates to a genetically modified plant comprising the genetically modified plant cell as described above.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate, by way of example only, embodiments of the present invention:

FIG. 1A-1D show nucleic acid sequence and amino acid sequence alignments between AP14 [SEQ ID NO:2 and 4] and PGO3 [SEQ ID NO:1 and 3], with Clustal W™ (1.83).

FIG. 2A-2E show cloned novel partial sequences and/or nucleic acid sequence fragments of known apple PPO isoenzyme encoding sequences: APO5 [SEQ ID NO:5]; PPO3 [SEQ ID NO: 6]; PPO7 [SEQ ID NO: 7]; PPO2 [SEQ ID NO:8]; PPOJ [SEQ ID NO:9]; GPO3 [SEQ ID NO:10]; AP14 [SEQ ID NO:11]; pSR7 [SEQ ID NO:12]; APO3 5′ [SEQ ID NO:13]; APO9 5′ [SEQ ID NO:14]; APO3 3′ [SEQ ID NO:15]; APO9 3′ [SEQ ID NO:16]; and pSR8 [SEQ ID NO:17].

FIG. 3 shows sequence identity (%) obtained through a multiple nucleic acid sequence alignments of apple PPO encoding sequences with Clustal W™ (1.82).

FIG. 4A-4D show nucleic acid sequence and amino acid sequence of four apple PPO encoding sequences: PPO2 [SEQ ID No:18] PPO2 (translation) [SEQ ID NO:19]; GPO3 [SEQ ID NO:20]; GPO3 (translation) [SEQ ID NO:21]; APO5 [SEQ ID NO:22]; APO5 (translation) [SEQ ID NO:23]; pSR7 [SEQ ID NO:24]; and pSR7 (translation) [SEQ ID NO:25].

FIG. 5A-5B: A. shows nucleic acid fragment of four PPO encoding sequences used for constructing PGAS: PPO2 [SEQ ID NO:26]; GPO3 [SEQ ID NO:27]; APO5 [SEQ ID NO:28]; and PSR7 [SEQ ID NO:29]. B. shows the nucleic acid fragments used in the construction of PGAS2: PPO2 [SEQ ID NO:30]; GPO3 [SEQ ID NO:31]; APO5 [SEQ ID NO:32]; PSR7 [SEQ ID NO:33]; and ACO2 [SEQ ID NO:34].

FIG. 6A-6B. A. shows the nucleic acid sequence of the PGAS transgene fragment in the sense orientation [SEQ ID NO:35]. B. shows the nucleic acid sequence of the PGAS2 transgene fragment [SEQ ID NO:36].

FIG. 7A-7X. A-L show nucleic acid sequence alignment between the apple PPO isoenzyme encoding genomic sequences and the corresponding nucleic acid sequences used for constructing PGAS: PPO2 [SEQ ID NO:37] and PPO2_PGAS [SEQ ID NO:38]; GPO3 [SEQ ID NO:39] and GPO3_PGAS [SEQ ID NO:40]; APO5 [SEQ ID NO:41] and APO5_PGAS [SEQ ID NO:42]; pSR7 [SEQ ID NO:43] and pSR7_PGAS [SEQ ID NO:44]. M-X show nucleic acid sequence alignment between the apple PPO isoenzyme encoding genomic sequences and the corresponding nucleic acid sequences used for constructing PGAS2: PPO2 [SEQ ID NO:45] and PPO2_PGAS2 [SEQ ID NO:46]; GPO3 [SEQ ID NO:47] and GPO3_PGAS2 [SEQ ID NO:48]; APO5 [SEQ ID NO:49] and APO5_PGAS2 [SEQ ID NO:50]; pSR7 [SEQ ID NO:51] and pSR7_PGAS2 [SEQ ID NO:52].

FIG. 8A-8B: A. shows an illustrative representation of GEN-03 comprising the PGAS transgene fragment in the sense orientation; and B. illustrates OSF-2, where P=PPO2; G=GPO3; A=APO5; and S=pSR7.

FIG. 9A-9E: A-C show nucleic acid sequence of the T-DNA elements of GEN-03 which are typically transferred into a plant following a transformation event: LB [SEQ ID NO:53]; P_(NOS) [SEQ ID NO:54]; nptII [SEQ ID NO:55]; T_(NOS) [SEQ ID NO:56]; P_(CAMV35S) [SEQ ID NO:57]; PGAS [SEQ ID NO:58] T_(NOS) [SEQ ID NO:59]; and RB [SEQ ID NO:60]. D-E shows the nucleic acid sequence of the elements for PGAS2: RB [SEQ ID NO:61]; P_(BUL409S) [SEQ ID NO:62]; PGAS2 [SEQ ID NO:63]; T_(UBI3) [SEQ ID NO:64]; and LB [SEQ ID NO:65].

FIG. 10A-10D show PPO suppression or activity in examples of GEN-03 (A and B) and OSF-02 (C and D) genetic events.

FIG. 11 shows an illustrative example of the results obtained from a detailed examination of the reduced-browning phenotype of genetic events sent to field trial.

FIG. 12 shows an illustrative example of a controlled bruised response of genetic events (743 and 784) sent to field trial relative to control events.

FIG. 13 shows an illustrative example of a reduced-browning phenotype of juice produced from genetic events (743 and 784) sent to field trial relative to control events.

FIG. 14A-14C show an illustrative example of measurement of PPO gene expression, total PPO activity, and change in luminosity in immature fruit obtained from genetic events sent to field trial, in two independent experiments (I and II).

FIG. 15 shows the nucleic acid sequence of three pear PPO encoding sequences: PearGB [SEQ ID NO:66]; Pear1 [SEQ ID NO:67] and Pear2 [SEQ ID NO:68].

FIG. 16 shows sequence identity (%) obtained through a multiple nucleic acid sequence alignment of apple and pear PPO encoding sequences with Clustal W™ (1.82).

FIG. 17 shows the relationship between PPO activity in tissue culture leaf material (TC), PPO activity in immature fruit tissue (2005), Impact Bruising (2005 and 2006) and PPO gene expression in immature fruit (2005).

DETAILED DESCRIPTION

The invention provides a genetically modified fruit-producing plant, seed, seedling, progeny thereof, or produced fruit thereof, which genetically modified plant produces a reduced-browning fruit by having a reduced Polyphenol Oxidase (PPO) activity relative to control fruit-producing plant, seed, seedling, progeny thereof, or produced fruit thereof. The invention also provides a method of obtaining such genetically modified fruit-producing plant.

Polyphenol Oxidase (PPO) Expression in Plants

PPOs are copper-containing metalloenzymes that catalyze the oxidation of phenols to produce quinones. Quinones may subsequently react with amino acids and proteins to form brown and black pigments, resulting in the browning of produce, including without limitation, fruits and vegetables. PPOs are localized to the plastid in plants, whereas the phenolic substrates of the enzymes are sequestered in vacuoles. This compartmentalization prevents a PPO from reacting with its substrate unless the plant cells are damaged and the enzyme and its substrate are mixed.

PPO gene expression has been described in diverse genera of animals, plants, fungi and bacteria (for a review, see Mayer (2006)).

Any plant that expresses PPO and is susceptible to bruising, or produces fruit that is susceptible to bruising may be used in the context of the invention.

Among plants, PPO has been described, without limitation, in fruit-producing plants (e.g. apricot, apple, banana, pear), vegetables (e.g. artichoke, cabbage, potato, tomato), flowers (e.g. orchid); herbs (e.g. oregano); grains (e.g. wheat); and others (e.g. red clover).

The invention has application in any fruit-producing plant that expresses PPO. Such plants include, without limitation, apple, apricot, avocado, banana, blackberry, blueberry, cherry, cranberry, custard apple, date, durian, fig, grapefruit, grape, jack fruit, kiwi fruit, lychee, mandarin, mangosteen, mango, melon, nashi, nectarine, orange, papaya or paw paw, passionfruit, peach, pear, persimmon, pineapple, plum, pomegranate, pomelo, raspberry, rhubarb, star fruit, strawberry, tamarillo, and tangerine plants or trees (as used herein the term “plant” is intended to encompass also fruit-producing trees). The invention encompasses also cells, seeds, seedlings, parts, tissues, fruit and progeny of such plants.

In one embodiment the plant is an apple plant. The following are examples of PPO genes described in apple, and which may be targeted for reduction of PPO activity:

U.S. Pat. No. 6,936,748 (Robinson and Dry) described the cloning of PPO genes from potato tubers, grape, apple and broad bean. Apple PPO genes pSR7 and pSR8 were identified in addition to a partial sequence of a genomic clone, GALPO3, which appears to be very similar to the apple PPO gene, APO3.

Boss et al. (1995) screened an apple cDNA library with pSR8 and isolated six clones, including the PPO gene APO5. In personal communications with Boss, it was noted that other apple PPO clones, APO1, APO2, APO3 and APO9, were similar or identical to each other and 70% identical to APO5.

Haruta et al. (1998) isolated and characterized two apple PPO clones PPO3 and PPO7 that are nearly identical to APO5.

Kim et al. (2001) teaches that apple PPO gene PPO2 is 96% identical to pSR8. PPO2 has less homology with the other apple isoenzymes than they have to each other. PPO2 is not closely related to peach or cherry PPOs but somewhat related to a PPO sequence from apricot.

In many plant species, PPO genes are organized in multigene families. For example, Kruger et al (1976) reported 12 isoenzymes of PPO in wheat. Newman et al (1993) reported at least six PPO genes in tomato, with homologies ranging from 70-96%. Boss et al (1995) reported at least four PPO genes in apple.

The invention involves reducing activity of at least two PPO isoenzymes in a fruit-producing plant. As used herein, the term “PPO isoenzyme” encompasses enzymes that differ in amino acid sequence but catalyze the same chemical reaction. In biochemistry, isoenzymes (or isozymes) are isoforms (closely related variants) of enzymes. In many cases, homologous genes encode isoenzymes.

In an embodiment, the plant is an apple plant (Malus×domestica). The invention may be practiced in any variety of apple, such as, for example, Golden Delicious, Granny Smith, Fuji, Gala, MacIntosh, PPO isoenzymes in apple, the activity of which may be reduced in accordance with the invention, include, without limitation, any two or more of APO5 (Boss et al. (1995); pSR7; (Robinson (1993)); pSR8 (PPO2) (Robinson (1993)); PPO2 (HortResearch; Kim et al. (2001)); GPO3 (HortResearch, Boss), APO; PPO3 and PPO7 (Haruta et al. (1998).

In an embodiment, PPO2 comprises, consists of, or consists essentially of the amino acid sequence set forth in SEQ ID NO: 19. In an embodiment, GPO3 comprises, consists of, or consists essentially of the amino acid sequence set forth in SEQ ID NO: 21. In an embodiment, APO5 comprises, consists of, or consists essentially of the amino acid sequence set forth in SEQ ID NO: 23. In an embodiment, PSR7 comprises, consists of, or consists essentially of the amino acid sequence set forth in SEQ ID NO 25.

It is anticipated that some apple varieties or indeed even other fruit-producing plant species may contain variants of PPO2, GPO3, APO5 and PSR7 that may be targeted for reduction in activity in accordance with the invention. Accordingly, the PPO isoenzyme may have an amino acid sequence that possesses at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity to SEQ ID NO: 19, 21, 23 or 25. Preferably such variant sequences are species or allelic variants of the PPO2, GPO3, APO5 or PSR7 sequence as set forth in SEQ ID NO: 19, 21, 23 and 25, respectively.

The PPO isoenzyme may also be a fragment of a PPO isoenzyme as described above, the fragment possessing PPO activity. Such fragments may comprise e.g. at least 10, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 150, at least 200, at least 250, or at least 300 amino acids.

In an embodiment, the PPO isoenzyme described herein is a polypeptide that retains at least some PPO activity of any one of the apple isoenzymes APO5, GPO3, PPO2 or pSR7 but differs in sequence from any one of these by one or more amino acid insertions, deletions, or substitutions, particularly conservative amino acid substitutions. As used herein, the expression “conservative amino acid substitutions” refers to the substitution of one amino acid for another at a given location in the polypeptide, where the substitution occurs without substantial loss of the relevant function. Conservative substitutions generally involve substitution of an amino acid residue with another amino acid residue on the basis of relative similarity of side-chain substituents, for example, their size, charge, hydrophobicity, hydrophilicity, and the like. Specific, non-limiting examples of a conservative substitution include the following examples:

Original Residue Conservative Substitutions Ala Ser Arg Lys Asn Gln, His Asp Glu Cys Ser Gln Asn Glu Asp His Asn; Gln Ile Leu, Val Leu Ile; Val Lys Arg; Gln; Glu Met Leu; Ile Phe Met; Leu; Tyr Ser Thr Thr Ser Trp Tyr Tyr Trp; Phe Val Ile; Leu

As used herein, the term “polypeptide” encompasses any chain of naturally or non-naturally occurring amino acids (either D- or L-amino acids), regardless of length (e.g., at least 5, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50, 100 or more amino acids) or post-translational modification (e.g., glycosylation or phosphorylation) or the presence of e.g. one or more non-amino acyl groups (for example, sugar, lipid, etc.) covalently linked to the peptide, and includes, for example, natural proteins, synthetic or recombinant polypeptides and peptides, hybrid molecules, peptoids, peptidomimetics, etc. As used herein, the terms “polypeptide”, “peptide” and “protein” may be used interchangeably.

Other apple PPO isoenzymes may be identified for use in the context of the invention. As disclosed herein, a degenerate primer approach was used to identify novel PPO gene sequences. Other approaches to identify novel PPO isoenzymes are known in the art include, but not limited to, use of degenerate primers to screen an expression library from plants that produce fruit susceptible to browning; and the use of bioinformatics to virtually identify other PPO genes. Following the identification of a candidate PPO gene, it may be disrupted in a fruit producing plant and the produced fruit may be assessed for altered fruit-browning properties, with any of the approaches disclosed herein.

Other approaches to identify PPO isoenzymes include, without limitation, screening cDNA, genomic or bac libraries with PPO probes from apple or other species. Alternatively, sequences could be identified in the apple genome sequence soon to be published (IASMA—Istituto Agrario San Michele all'Adige).

When the invention is practiced in a pear plant, non-limiting examples of PPO isoenzymes that may be targeted for reduced activity include without limitation PPOs encoded by PearGB [SEQ ID NO:66]; Pear1 [SEQ ID NO:67] and Pear2 [SEQ ID NO:68], or fragments or variants thereof as described above.

Reduction of PPO Activity or Expression

PPO expression and/or activity in genetically modified plants of the present invention may be reduced by any method that results in reduced activity of at least two PPO isoenzymes in the plant. This may be achieved by e.g. by altering PPO at the DNA, mRNA and/or protein levels.

As used herein, “activity” refers to the biochemical reaction of an enzyme with its cognate substrate. In the context of the invention, reduced PPO activity may result from reduced protein levels of a PPO isoenzyme and/or the reduced rate at which a PPO isoenzyme catalyzes its reaction with a substrate.

Total PPO activity may be determined, without limitation, by using the polyphenol oxidase specific activity assay of Broothaerts et al (2000), or a modification thereof, for example, the assay adapted for use in microtitre plate format. PPO specific activity may be expressed in terms of U/mg protein. Substrates that may be used in the assay to determine PPO specific activity are known in the art, and include, without limitation, 4-methyl catechol.

In one embodiment, PPO expression and/or activity may be altered by targeting genomic PPO genes. For example, the endogenous PPO gene may be altered by, without limitation, knocking-out one or more PPO genes; or knocking-in a heterologous DNA to disrupt one or more PPO genes. The skilled person would understand that these approaches may be applied to the coding sequences, the promoter or other regulatory elements necessary for gene transcription.

In another embodiment, PPO expression and/or activity may be altered by targeting PPO mRNA transcripts. In this regard, levels of PPO mRNA transcripts may be reduced by methods known in the art including, but not limited to, co-suppression, antisense expression, small hair pin (shRNA) expression (Zhao et al.), interfering RNA (RNAi) expression (Matzke et al.), double stranded (dsRNA) expression (Karkare et al.), inverted repeat dsRNA expression (Otani et al.), micro interfering RNA (miRNA) (Willmann and Poethig, or Pikaard), simultaneous expression of sense and antisense sequences (Karkare et al.), or a combination thereof, targeted at least two PPO isoenzymes encoding genes.

The phenomenon of co-suppression in plants relates to the introduction of transgenic copies of a gene resulting in reduced expression of the transgene as well as the endogenous gene (Napoli et al (1990); van der Krol et al. (1990). The observed effect depends on sequence identity between the transgene and the endogenous gene.

RNA interference/silencing relates to the silencing of genes by the introduction of double stranded RNA. RNA is both an initiator and target in the process (Fire et al, (1998); Lindbo et al, (1993); Montgomery et al, (1998)). This mechanism targets RNA from viruses and transposons and also plays a role in regulating development and genome maintenance. Briefly, double stranded RNA is cleaved by the enzyme dicer resulting in short fragments of 21-23 bp (siRNA). One of the two strands of each fragment is incorporated into the RNA-induced silencing complex (RISC). The RISC associated RNA strand pairs with mRNA and induces cleavage of the mRNA. Alternatively, RISC associated RNA strand pairs with genomic DNA resulting in epigenetic changes that affect gene transcription. Micro RNA (miRNA) is a type of RNA transcribed from the genome itself and works in a similar way. Similarly, shRNA may be cleaved by dicer and associate with RISC resulting in mRNA cleavage.

Antisense suppression of gene expression does not involve the catalysis of mRNA degradation, but instead involves single-stranded RNA fragments binding to mRNA and blocking protein translation.

Both antisense and sense suppression are mediated by silencing RNAs (sRNAs) produced from either a sense-antisense hybrid or double stranded RNA (dsRNA) generated by an RNA-dependant RNA polymerase (Jorgensen 2006). Majors classes or sRNAs include short-interfering RNAs (siRNAs) and microRNAs (miRNAs) which differ in their biosynthesis.

Processing of dsRNA precursors by Dicer-Like complexes yields 21-nucleotide siRNAs and miRNAs guide cleavage of target transcipts from within RNA-induced silencing complexes (RISC).

Preferably PPO expression may be suppressed using an synthetic gene or an unrelated gene that contained about 21 bp regions of high homology (preferably 100% homology) to the PPO gene.

See, for example, Jorgensen R A, Doetsch N, Muller A, Que Q, Gendler, K and Napoli C A (2006) A paragenetic perspective on integration of RNA silencing into the epigenome and in the biology of higher plants. Cold Spring Harb. Symp. Quant. Biol. 71:481-485.

For a review, see for example, Ossowski S, Schwab R and Weigel D (2008) Gene silencing in plants using artificial microRNAs and other small RNAs. The Plant Journal 53:674-690.

In a further embodiment, PPO activity may be altered by targeting one or more PPO isoenzymes at the protein level. For example, a PPO isoenzyme activity may be altered by affecting the post-translational modification of the enzyme; or by the introduction of a heterologous protein (e.g. a mutated form of one or more PPO isoenzymes may be expressed such that it associates with the wildtype PPO isoenzyme and alters its activity; or an antibody that binds specifically to one or more PPO isoenzymes).

As used herein, “expression” or “expressing” refers to production of any detectable level of a product encoded by the coding sequence. In the context of the invention, reduced PPO expression may result from reduced transcription of a PPO gene or from reduced translation of PPO mRNA transcripts.

In one embodiment, a genetically modified fruit producing plant of the invention comprises, stably integrated into its genome a first nucleic acid molecule heterologous to the plant, the presence of the first nucleic acid molecule reducing expression of a first PPO isoenzyme; and a second nucleic acid molecule heterologous to the plant, the presence of the second nucleic acid molecule in the plant reducing expression of a second PPO isoenzyme.

In one embodiment, a genetically modified plant of the present invention may further comprise a third nucleic acid molecule heterologous to the plant, the presence of said third nucleic acid molecule reducing expression of a third PPO isoenzyme.

In another embodiment, the genetically modified plant of the present invention may further comprise a fourth nucleic acid molecule heterologous to the plant, the presence of the fourth nucleic acid molecule reducing expression of a fourth PPO isoenzyme.

If a plant expresses additional PPO isoenzymes, additional heterologous nucleic acid molecules (e.g. fifth, sixth, seventh and eighth heterologous nucleic acid molecules) may also be used to reduce expression of such PPO isoenzymes.

A nucleic acid molecule that reduces the expression and/or activity of any PPO isoenzyme may be used in the context of the invention. For instance, suitable apple PPO isoenzymes that may be targeted in apple plants to reduce browning in the fruit of the plant include those described above.

Further, a nucleic acid sequence comprising a nucleic acid sequence that is at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to the nucleic acid sequence of any known apple or other fruit PPO isoenzyme may be suitable for use in the context of the invention.

Fragments of nucleic acid sequences encoding fruit PPO isoenzymes may be used. Such fragments may have lengths of at least 20, at least 50, at least 100, at least 150, at least 200, at least 300 or at least 400 contiguous nucleotides of a nucleic acid sequence encoding a PPO. Alternatively such fragments may have a minimum length of at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 contiguous nucleotides and a maximum length less than 3000, less than 2000, less than 1750, less than 1500, less than 1250, less than 1000, less than 750 or less than 500 contiguous nucleotides or any combination of such minimum and maximum lengths of a nucleic acid sequence encoding a PPO.

In one aspect, a nucleic acid molecule may comprise PPO2 (SEQ ID NO: 18); GPO3 (SEQ ID NO: 20); APO5 (SEQ ID NO: 22); pSR7 (SEQ ID NO: 24); a species variant thereof, an allelic variant thereof; a non-natural variant thereof; or a fragment thereof.

In one embodiment, a fragment of PPO2 as set forth in SEQ ID NO: 26; a fragment of GPO3 as set forth in SEQ ID NO: 27; a fragment of APO5 as set forth in SEQ ID NO: 28; and a fragment of pSR7 as set forth in SEQ ID NO: 29 may be suitable for use in the context of the invention.

In another embodiment, a fragment of PPO2 as set forth in SEQ ID NO: 30; a fragment of GPO3 as set forth in SEQ ID NO: 31; a fragment of APO5 as set forth in SEQ ID NO: 32; and a fragment of pSR7 as set forth in SEQ ID NO: 33 may be suitable for use in the context of the invention.

The gene fragments PPO2, GPO3, APO5, and pSR7 described herein are merely illustrative. Gene fragments suitable for use in the context of the invention may differ in sequence, in length and in location relative to those noted above may be suitable for use in the context of the invention.

For example, a gene fragment (such as a fragment of e.g. PPO2, GPO3, APO5, or pSR7) may comprise at least 20, at least 40, at least 60, at least 80, at least 100, at least 150, at least 200, at least 150, at least 300, at least 350, at least 400, at least 450 or at least 500 contiguous nucleotides of said genes. A gene fragment of PPO2, GPO3, APO5 and/or pSR7 may be 5′ or 3′ of the fragments of those genes disclosed herein.

Nucleic acid molecules that are substantially identical to the PPO2, GPO3, APO5, and pSR7 genes disclosed herein, may also be used in the context of the invention. As used herein, one nucleic acid molecule may be “substantially identical” to another if the two molecules have at least 60%, at least 70%, at least 80%, at least 82.5%, at least 85%, at least 87.5%, at least 90%, at least 92.5%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity. In one embodiment, the two nucleic acid molecules each comprise at least 20 identical contiguous nucleotides.

In various embodiments of the invention, the at least two heterologous nucleic acid molecules are selected from:

at least 20, at least 50, at least 100, at least 150, at least 200, at least 300 or at least 400 contiguous nucleotides of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 18;

at least 20, at least 50, at least 100, at least 150, at least 200, at least 300 or at least 400 contiguous nucleotides of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 20;

at least 20, at least 50, at least 100, at least 150, at least 200, at least 300 or at least 400 contiguous nucleotides of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 22; and

at least 20, at least 50, at least 100, at least 150, at least 200, at least 300 or at least 400 contiguous nucleotides of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic sequence set forth in SEQ ID NO: 24.

In other embodiments of the invention, the at least two heterologous nucleic acid molecules are selected from:

a nucleic acid molecule with a minimum length of at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 contiguous nucleotides and a maximum length less than 1750, less than 1500, less than 1250, less than 1000, less than 750 or less than 500 contiguous nucleotides or any combination of such minimum and maximum lengths of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 18;

a nucleic acid molecule with a minimum length of at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 contiguous nucleotides and a maximum length less than 1750, less than 1500, less than 1250, less than 1000, less than 750 or less than 500 contiguous nucleotides or any combination of such minimum and maximum lengths of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 20;

a nucleic acid molecule with a minimum length of at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 contiguous nucleotides and a maximum length less than 1750, less than 1500, less than 1250, less than 1000, less than 750 or less than 500 contiguous nucleotides or any combination of such minimum and maximum lengths of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 22; and

a nucleic acid molecule with a minimum length of at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 contiguous nucleotides and a maximum length less than 1750, less than 1500, less than 1250, less than 1000, less than 750 or less than 500 contiguous nucleotides or any combination of such minimum and maximum lengths of a nucleic acid sequence possessing at least 80%, at least 90% or 100% sequence identity to the nucleic sequence set forth in SEQ ID NO: 24.

The term “identity” refers to sequence similarity between two polypeptide or polynucleotide molecules. Identity can be determined by comparing each position in the aligned sequences. A degree of identity between amino acid or nucleic acid sequences is a function of the number of identical or matching amino acids or nucleic acids at positions shared by the sequences, for example, over a specified region. Optimal alignment of sequences for comparisons of identity may be conducted using a variety of algorithms, as are known in the art, including the Clustal W™ program, available at http://clustalw.genome.ad.jp, the local homology algorithm of Smith and Waterman, 1981, Adv. Appl. Math 2: 482, the homology alignment algorithm of Needleman and Wunsch, 1970, J. Mol. Biol. 48:443, the search for similarity method of Pearson and Lipman, 1988, Proc. Natl. Acad. Sci. USA 85:2444, and the computerised implementations of these algorithms (such as GAP, BESTFIT, FASTA and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, Madison, Wis., U.S.A.). Sequence identity may also be determined using the BLAST algorithm (e.g. BLASTn and BLASTp), described in Altschul et al., 1990, J. Mol. Biol. 215:403-10 (using the published default settings). Software for performing BLAST analysis is available through the National Center for Biotechnology Information (through the Internet at http://www.ncbi.nlm.nih.gov/). For instance, sequence identity between two nucleic acid sequences can be determined using the BLASTn algorithm at the following default settings: expect threshold 10; word size 11; match/mismatch scores 2, −3; gap costs existence 5, extension 2. Sequence identity between two amino acid sequences may be determined using the BLASTp algorithm at the following default settings: expect threshold 10; word size 3; matrix BLOSUM 62; gap costs existence 11, extension 1. In another embodiment, the person skilled in the art can readily and properly align any given sequence and deduce sequence identity/homology by mere visual inspection.

In the alternative, two nucleic acid sequences encoding PPO isoenzymes may be substantially complementary (or are homologues/have identity) if the two sequences hybridize to each other under moderately stringent, or preferably stringent, conditions. Hybridization to filter-bound sequences under moderately stringent conditions may, for example, be performed in 0.5 M NaHPO₄, 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.2×SSC/0.1% SDS at 42° C. (see Ausubel, et al. (eds), 1989, Current Protocols in Molecular Biology, Vol. 1, Green Publishing Associates, Inc., and John Wiley & Sons, Inc., New York, at p. 2.10.3). Alternatively, hybridization to filter-bound sequences under stringent conditions may, for example, be performed in 0.5 M NaHPO₄, 7% SDS, 1 mM EDTA at 65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. (see Ausubel, et al. (eds), 1989, supra). Hybridization conditions may be modified in accordance with known methods depending on the sequence of interest (see Tijssen, 1993, Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes, Part I, Chapter 2 “Overview of principles of hybridization and the strategy of nucleic acid probe assays”, Elsevier, N.Y.). Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point for the specific sequence at a defined ionic strength and pH.

The first, second, third and/or fourth or additional nucleic acid molecules may be present in a single genetic construct or in multiple constructs. In one embodiment, the first, second, third and/or fourth or additional nucleic acid molecules may be arranged in the sense orientation relative to a promoter. In another embodiment, the first, second, third and/or fourth or additional nucleic acid molecules may be arranged in the anti-sense orientation relative to a promoter. In a further embodiment, a genetic construct may comprise at least two nucleic acid molecules in both the sense and anti-sense orientations, relative to a promoter. A genetic construct comprising nucleic acids in both the sense and anti-sense orientations may result in mRNA transcripts capable of forming stem-loop structures.

One or more of the nucleic acid molecules may be under transcriptional control of the same promoter.

A genetic construct comprising nucleic acids in both orientations relative to a promoter may further comprise a spacer to separate the nucleic acid molecules in sense orientation and those in the anti-sense orientation. As used herein, a “spacer” may comprise at least 2, at least 5, at least 10, at least 20, at least 30, at least 40, at least 50, at least 75, at least 100, at least 150, or at least 200 nucleotides. In one embodiment, the spacer may be an intron, such as an intron from a PPO gene.

In the context of the present invention, the nucleic acid molecules may comprise nucleic acid that is heterologous to the plant in which PPO activity is reduced. As used herein, “heterologous”, “foreign” and “exogenous” DNA and RNA are used interchangeably and refer to DNA or RNA that does not occur naturally as part of the plant genome in which it is present or which is found in a location or locations in the genome that differ from that in which it occurs in nature. Thus, heterologous or foreign DNA or RNA is nucleic acid that is not normally found in the host genome in an identical context (i.e. linked to identical 5′ and 3′ sequences). In one aspect, heterologous DNA may be the same as the host DNA but introduced into a different place in the host genome and/or has been modified by methods known in the art, where the modifications include, but are not limited to, insertion in a vector, linked to a foreign promoter and/or other regulatory elements, or repeated at multiple copies. In another aspect, heterologous DNA may be from a different organism, a different species, a different genus or a different kingdom, as the host DNA. Further, the heterologous DNA may be a transgene. As used herein, “transgene” refers to a segment of DNA containing a gene sequence that has been isolated from one organism and introduced into a different organism.

As used herein, “nucleotide sequence”, “polynucleotide sequence”, “nucleic acid” or “nucleic acid molecule” may refer to a polymer of DNA or RNA which can be single or double stranded and optionally containing synthetic, non-natural or altered nucleotide bases capable of incorporation into DNA or RNA polymers. “Nucleic acid”, “nucleic acid sequence”, “polynucleotide sequence” or “nucleic acid molecule” may encompass genes, cDNA, DNA and RNA encoded by a gene. Nucleic acids, nucleic acid sequences, polynucleotide sequence and nucleic acid molecule may comprise at least 3, at least 10, at least 100, at least 1000, at least 5000, or at least 10000 nucleotides or base pairs.

As used herein, a “fragment”, a “fragment thereof”, “gene fragment” or a “gene fragment thereof” refers to a portion of a “nucleotide sequence”, “polynucleotide sequence”, “nucleic acid” or “nucleic acid molecule” that may still reduce total PPO gene expression and/or fruit browning. In one embodiment, the fragment comprises at least 20, at least 40, at least 60, at least 80, at least 100, at least 150, at least 200, at least 150, at least 300, at least 350, at least 400, at least 450 or at least 500 contiguous nucleotides.

As used herein, a “non-natural variant” refers to nucleic acid sequences native to an organism but comprising modifications to one or more of its nucleotides. Nucleic acids may be modified by any chemical and/or biological means known in the art including, but not limited to, reaction with any known chemicals such as alkylating agents, browning sugars, etc; conjugation to a linking group (e.g. PEG); methylation; oxidation; ionizing radiation; or the action of chemical carcinogens. Such nucleic acid modifications may occur during synthesis or processing or following treatment with chemical reagents known in the art.

As used herein, a “species variant” refers to an alternate form of the same PPO gene as found in different species of the same genus. The term may also refer to an alternate form of the same PPO gene as found in different varieties of the same species, for example, of a plant.

As used herein, an “allelic variant” refers to an alternate form of the same gene at a specific location of the genome.

As used herein, “wildtype” may refer to a plant or plant material that was not transformed with a nucleic acid molecule or construct, as described herein. A “wildtype” may also refer to a plant or plant material in which total PPO activity was not reduced from a reduction in activity of at least two PPO isoenzymes.

The person skilled in the art will also readily understand that although in the foregoing illustrative examples partial PPO coding sequences were used to construct the PPO suppression transgene, complete PPO coding sequences, alternative PPO coding sequences, 5′UTR and/or 3′UTR, or mutated derivatives of these sequences can also be used.

The skilled person would appreciate that the reduction in activity of at least two PPO isoenzymes may not be limited by the number of different nucleic acid molecules introduced into a plant or plant cell. In one embodiment, one nucleic acid molecule may target one or more PPO isoenzyme genes. For example, one nucleic acid molecule may target at least one, at least two, at least three, or more PPO isoenzyme genes. In another example, 3 gene segments may be used to effect suppression of 4 PPO isoenzyme gene targets: 1 segment specific for PPO2, 1 segment specific for pSR7, and 1 segment capable of targeting both APO5 and GPO3 (for example, due to microhomology). In another embodiment, one or more nucleic acid molecules may be used to target one PPO isoenzyme gene. In a further embodiment, one nucleic acid molecule may target one PPO isoenzyme gene.

The maximum number of nucleic acid molecules that may be used in the context of the invention may be limited only by the maximum size of the construct that may be delivered to a target plant or plant cell using a given transformation method.

The skilled person would also appreciate that a nucleic acid molecule comprising the sequence of a PPO gene promoter and/or other regulatory elements may be used in the context of the invention. In an embodiment, a heterologous nucleic acid molecule comprising sequences of a PPO gene promoter and/or regulatory element may be used to bias the cellular machinery away from an endogenous PPO gene promoter thus resulting in reduced PPO gene expression.

Suppression Construct

A construct of the invention comprising a first, second, third and/or fourth nucleic acid molecule may further comprise a promoter and other regulatory elements, for example, an enhancer, a silencer, a polyadenylation site, a transcription terminator, a selectable marker or a screenable marker.

As used herein, a “vector” or a “construct” may refer to any recombinant polynucleotide molecule such as a plasmid, cosmid, virus, vector, autonomously replicating polynucleotide molecule, phage, or linear or circular single-stranded or double-stranded DNA or RNA polynucleotide molecule, derived from any source. A “vector” or a “construct” may comprise a promoter, a polyadenylation site, an enhancer or silencer and a transcription terminator, in addition to a nucleotide sequence encoding a gene or a gene fragment of interest. As used herein, a “transformation vector” may refer to a vector used in the transformation of, or in the introduction of DNA into, cells, plants or plant materials.

As used herein, a “promoter” refers to a nucleotide sequence that directs the initiation and rate of transcription of a coding sequence (reviewed in Roeder, Trends Biochem Sci, 16: 402, 1991). The promoter contains the site at which RNA polymerase binds and also contains sites for the binding of other regulatory elements (such as transcription factors). Promoters may be naturally occurring or synthetic (see Datla et al. Biotech Ann. Rev 3:269, 1997 for review of plant promoters). Further, promoters may be species specific (for example, active only in B. napus); tissue specific (for example, the napin, phaseolin, zein, globulin, dlec2, γ-kafirin seed specific promoters); developmentally specific (for example, active only during embryogenesis); constitutive (for example maize ubiquitin, rice ubiquitin, rice actin, Arabidopsis actin, sugarcane bacilliform virus, CsVMV and CaMV 35S, Arabidopsis polyubiquitin, Solanum bulbocastanum polyubiquitin, Agrobacterium tumefaciens-derived nopaline synthase, octopine synthase, and mannopine synthase gene promoters); or inducible (for example the stilbene synthase promoter and promoters induced by light, heat, cold, drought, wounding, hormones, stress and chemicals). A promoter includes a minimal promoter that is a short DNA sequence comprised of a TATA box or an Inr element, and other sequences that serve to specify the site of transcription initiation, to which regulatory elements are added for control of expression. A promoter may also refer to a nucleotide sequence that includes a minimal promoter plus DNA elements that regulates the expression of a coding sequence, such as enhancers and silencers. Thus in one aspect, the expression of the constructs of the present invention may be regulated by selecting a species specific, a tissue specific, a development specific or an inducible promoter.

Enhancers and silencers are DNA elements that affect transcription of a linked promoter positively or negatively, respectively (reviewed in Blackwood and Kadonaga, Science, 281: 61, 1998).

Polyadenylation site refers to a DNA sequence that signals the RNA transcription machinery to add a series of the nucleotide A at about 30 bp downstream from the polyadenylation site.

Transcription terminators are DNA sequences that signal the termination of transcription. Transcription terminators are known in the art. The transcription terminator may be derived from Agrobacterium tumefaciens, such as those isolated from the nopaline synthase, mannopine synthase, octopine synthase genes and other open reading frame from Ti plasmids. Other terminators may include, without limitation, those isolated from CaMV and other DNA viruses, dlec2, zein, phaseolin, lipase, osmotin, peroxidase, PinII and ubiquitin genes, for example, from Solanum tuberosum.

In the context of the invention, the nucleic acid construct may further comprise a selectable marker. Selectable markers may be used to select for plants or plant cells that contain the exogenous genetic material. The exogenous genetic material may include, but is not limited to, an enzyme that confers resistance to an agent such as a herbicide or an antibiotic, or a protein that reports the presence of the construct.

Numerous plant selectable marker systems are known in the art and are consistent with this invention. The following review article illustrates these well known systems: Miki and McHugh; Journal of Biotechnology 107: 193-232; Selectable marker genes in transgenic plants: applications, alternatives and biosafety (2004).

Examples of a selectable marker include, but are not limited to, a neo gene, which codes for kanamycin resistance and can be selected for using kanamycin, NptII, G418, hpt etc.; an amp resistance gene for selection with the antibiotic ampicillin; an hygromycinR gene for hygromycin resistance; a BAR gene (encoding phosphinothricin acetyl transferase) which codes for bialaphos resistance including those described in WO/2008/070845; a mutant EPSP synthase gene, aadA, which encodes glyphosate resistance; a nitrilase gene, which confers resistance to bromoxynil; a mutant acetolactate synthase gene (ALS), which confers imidazolinone or sulphonylurea resistance, ALS, and a methotrexate resistant DHFR gene.

Further, screenable markers that may be used in the context of the invention include, but are not limited to, a β-glucuronidase or uidA gene (GUS), which encodes an enzyme for which various chromogenic substrates are known, green fluorescent protein (GFP), and luciferase (LUX).

The size or length of the nucleic acid construct or elements thereof, are not limited to the specific embodiments described herein. For example, the skilled person would appreciate that the size of a transgene element may be defined instead by transgene element function; and that the promoter element may be determined instead as one that was capable of driving transcription at a sufficient level and in the desired tissues. Similarly, the stem loop structure formed by the mRNA transcribed by a nucleic acid construct of the invention, may comprise a number of gene segments which may vary in length. For example, the stem loop may comprise 3 gene segments of about 21-30 basepairs each, in addition to a spacer, such as an intron (126 bp plus intron).

The skilled person would appreciate that the size of the gene segments may be established by the sum of the element sizes combined and may depend on the transformation method used to deliver the transgene into the target organism. For example, each transformation method (Agrobacterium, biolistics, VIGS-based delivery systems) may be limited to theoretical maximum transgene sizes.

Plant Transformation

The present invention is not limited to any particular method for transforming plant cells. Methods for introducing nucleic acids into cells (also referred to herein as “transformation”) are known in the art and include, but are not limited to: Viral methods (Clapp. Clin Perinatol, 20: 155-168, 1993; Lu et al. J Exp Med, 178: 2089-2096, 1993; Eglitis and Anderson. Biotechniques, 6: 608-614, 1988; Eglitis et al, Avd Exp Med Biol, 241: 19-27, 1988); physical methods such as microinjection (Capecchi. Cell, 22: 479-488, 1980), electroporation (Wong and Neumann. Biochim Biophys Res Commun, 107: 584-587, 1982; Fromm et al, Proc Natl Acad Sci USA, 82: 5824-5828, 1985; U.S. Pat. No. 5,384,253) and the gene gun (Johnston and Tang. Methods Cell Biol, 43: 353-365, 1994; Fynan et al. Proc Natl Acad Sci USA, 90: 11478-11482, 1993); chemical methods (Graham and van der Eb. Virology, 54: 536-539, 1973; Zatloukal et al. Ann NY Acad Sci, 660: 136-153, 1992); and receptor mediated methods (Curiel et al. Proc Natl Acad Sci USA, 88: 8850-8854, 1991; Curiel et al. Hum Gen Ther, 3: 147-154, 1992; Wagner et al. Proc Natl Acad Sci USA, 89: 6099-6103, 1992).

The introduction of DNA into plant cells by Agrobacterium mediated transfer is well known to those skilled in the art. If, for example, the Ti or Ri plasmids are used for the transformation of the plant cell, at least the right border, although more often both the right and the left border of the T-DNA contained in the Ti or Ri plasmid must be linked to the genes to be inserted as flanking region. If agrobacteria are used for the transformation, the DNA to be integrated must be cloned into special plasmids and specifically either into an intermediate or a binary vector. The intermediate vectors may be integrated into the Ti or Ri plasmid of the agrobacteria by homologous recombination due to sequences, which are homologous to sequences in the T-DNA. This also contains the vir-region, which is required for T-DNA transfer. Intermediate vectors cannot replicate in agrobacteria. The intermediate vector can be transferred to Agrobacterium tumefaciens by means of a helper plasmid (conjugation). Binary vectors are able to replicate in E. coli as well as in agrobacteria. They contain a selection marker gene and a linker or polylinker framed by the right and left T-DNA border region. They can be transformed directly into agrobacteria. The agrobacterium acting as host cell should contain a plasmid carrying a vir-region. The vir-region is required for the transfer of the T-DNA into the plant cell. Additional T-DNA may be present. Such a transformed agrobacterium is used for the transformation of plant cells. The use of T-DNA for the transformation of plant cells has been intensively studied and has been adequately described in standard review articles and manuals on plant transformation. Plant explants cultivated for this purpose with Agrobacterium tumefaciens or Agrobacterium rhizogenes can be used for the transfer of DNA into the plant cell.

Although Agrobacterium tumefaciens LBA4404 transformation has been described, a person skilled in the art will readily understand that any other suitable method of DNA transfer into plant may be used.

Another method for introducing DNA into plant cells is by biolistics. This method involves the bombardment of plant cells with microscopic particles (such as gold or tungsten particles) coated with DNA. The particles are rapidly accelerated, typically by gas or electrical discharge, through the cell wall and membranes, whereby the DNA is released into the cell and incorporated into the genome of the cell. This method is used for transformation of many crops, including corn, wheat, barley, rice, woody tree species and others. Biolistic bombardment has been proven effective in transfecting a wide variety of animal tissues as well as in both eukaryotic and prokaryotic microbes, mitochondria, and microbial and plant chloroplasts (Johnston. Nature, 346: 776-777, 1990; Klein et al. Bio/Technol, 10: 286-291, 1992; Pecorino and Lo. Curr Biol, 2: 30-32, 1992; Jiao et al, Bio/Technol, 11: 497-502, 1993).

Another method for introducing DNA into plant cells is by electroporation. This method involves a pulse of high voltage applied to protoplasts/cells/tissues resulting in transient pores in the plasma membrane which facilitates the uptake of foreign DNA. The foreign DNA enter through the holes into the cytoplasm and then to the nucleus.

Plant cells may be transformed by liposome mediated gene transfer. This method refers to the use of liposomes, circular lipid molecules with an aqueous interior, to deliver nucleic acids into cells. Liposomes encapsulate DNA fragments and then adhere to the cell membranes and fuse with them to transfer DNA fragments. Thus, the DNA enters the cell and then to the nucleus.

Other well-known methods for transforming plant cells which are consistent with the present invention include, but are not limited to, pollen transformation (See University of Toledo 1993 U.S. Pat. No. 5,177,010); Whiskers technology (See U.S. Pat. Nos. 5,464,765 and 5,302,523).

The nucleic acid constructs of the present invention may be introduced into plant protoplasts. Plant protoplasts are cells in which its cell wall is completely or partially removed using either mechanical or enzymatic means, and may be transformed with known methods including, calcium phosphate based precipitation, polyethylene glycol treatment and electroporation (see for example Potrykus et al., Mol. Gen. Genet., 199: 183, 1985; Marcotte et al., Nature, 335: 454, 1988). Polyethylene glycol (PEG) is a polymer of ethylene oxide. It is widely used as a polymeric gene carrier to induce DNA uptake into plant protoplasts. PEG may be used in combination with divalent cations to precipitate DNA and effect cellular uptake. Alternatively, PEG may be complexed with other polymers, such as poly(ethylene imine) and poly L lysine.

A nucleic acid molecule of the present invention may also be targeted into the genome of a plant cell by a number of methods including, but not limited to, targeting recombination, homologous recombination and site-specific recombination (see review Baszcynski et al. Transgenic Plants, 157: 157-178, 2003 for review of site-specific recombination systems in plants). Homologous recombination and gene targeting in plants (reviewed in Reiss. International Review of Cytology, 228: 85-139, 2003) and mammalian cells (reviewed in Sorrell and Kolb. Biotechnology Advances, 23: 431-469, 2005) are known in the art.

As used herein, “targeted recombination” refers to integration of a nucleic acid construct into a site on the genome, where the integration is facilitated by a construct comprising sequences corresponding to the site of integration.

Homologous recombination relies on sequence identity between a piece of DNA that is introduced into a cell and the cell's genome. Homologous recombination is an extremely rare event in higher eukaryotes. However, the frequency of homologous recombination may be increased with strategies involving the introduction of DNA double-strand breaks, triplex forming oligonucleotides or adeno-associated virus.

As used herein, “site-specific recombination” refers to the enzymatic recombination that occurs when at least two discrete DNA sequences interact to combine into a single nucleic acid sequence in the presence of the enzyme. Site-specific recombination relies on enzymes such as recombinases, transposases and integrases, which catalyse DNA strand exchange between DNA molecules that have only limited sequence homology. Mechanisms of site specific recombination are known in the art (reviewed in Grindley et al. Annu Rev Biochem, 75: 567-605, 2006). The recognition sites of site-specific recombinases (for example Cre and att sites) are usually 30-50 bp. The pairs of sites between which the recombination occurs are usually identical, but there are exceptions e.g. attP and attB of λ integrase (Landy. Ann Rev Biochem, 58: 913-949, 1989).

The nucleic acid molecule becomes stably integrated into the plant genome such that it is heritable to daughter cells in order that successive generations of plant cells have reduced PPO expression. This may involve the nucleic acid molecules of the present invention integrating, for instance integrating randomly, into the plant cell genome. Alternatively, the nucleic acid molecules of the present invention may remain as exogenous, self-replicating DNA that is heritable to daughter cells. As used herein, exogenous, self-replicating DNA that is heritable to daughter cells is also considered to be “stably integrated into the plant genome”.

Plant Culture

Plant cell culture techniques are known in the art (see for example Fischer et al. Biotechnol Appl Biochem, 30: 109-112, 1999; Doran. Current Opinions in Biotechnology, 11: 199-204, 2000). The skilled person would appreciate that the composition of the culture media, its pH and the incubating conditions, such as temperatures, aeration, CO₂ levels, and light cycles, may vary depending on the type of cells.

Plant Selection

After transformation, plant cells may be sub-cloned to obtain clonal populations of cells. Methods of sub-cloning cells are known in the art and include, but are not limited to, limiting dilution of the pool of transformed cells. For example, a construct of the invention may comprise a selectable or screenable marker, as described herein. A cell transformed with a construct comprising a selection marker may be grown under selective pressure to identify those that contain and/or express the construct.

Naturally, it could also be done without any selection marker, although this would involve a fairly high screening expenditure. If marker-free genetically modified plants are desired, there are also strategies available to the person skilled in the art, which allow subsequent removal of the marker gene, such as co-transformation and sequence-specific recombinases.

After preparing clonal populations of transgenic plant cells, the cells may be characterized and selected based on analysis at the level of DNA, RNA and protein. Preferably, transgenic plant cells in which the nucleic acid construct is stably integrated into the cell genome are selected. As used herein, “stably integrated” refers to the integration of genetic material into the genome of the transgenic plant cell and remains part of the plant cell genome over time. The term “stably integrated” may also refer to the persistence of an exogenous replicating DNA that is heritable to daughter cells.

Stable integration of nucleic acid constructs may be influenced by a number of factors including, but not limited to, the transformation method used and the vector containing the gene of interest. The transformation method determines which cell type can be targeted for stable integration. The type of vector used for stable integration defines the integration mechanism, the regulation of transgene expression and the selection conditions for stably expressing cells. After integration, the level and time of expression of the gene of interest may depend on the linked promoter and on the particular integration site.

Plant Regeneration

Once the plant material is transformed, it may be regenerated into plantlets or plants. Plant regeneration by tissue culture techniques is well established. For example, plant regeneration from cultured protoplasts is described in Evans et al, (1983); and Vasil I. R. (1986). Plants have been successfully micropropagated in vitro by organogenesis or somatic embryogenesis including, but not limited to, all major species of sugarcane, sugar beet, cotton, fruit and other trees, legumes and vegetables. The methods for regeneration vary from species to species of plants, but generally a suspension of transformed protoplasts containing copies of the heterologous gene is first provided. Callus tissue is formed and shoots may be induced from callus and subsequently rooted. Alternatively, embryo formation may be induced from a protoplast suspension. These embryos germinate to form mature plants. The culture media will generally contain various amino acids and hormones, such as auxin and cytokinins. Shoots and roots normally develop simultaneously. Efficient regeneration will depend on the type of explant, the physiological condition of the explant and physical and chemical media of the explant during culture, and on the history of the culture. In another alternative, plant material may be micrografted onto rootstocks.

Testing for Reduction of PPO Activity or Expression

Disruption of PPO genes, gene expression, or PPO enzymatic activity may be confirmed by methods known in the art of molecular biology (see e.g. Maniatis et al., (1989)). For example, disruption of PPO genes may be assessed by PCR followed by Southern blot analysis. PPO mRNA levels may, for example, be measured by real time PCR, RT-PCR, Northern blot analysis, micro-array gene analysis, and RNAse protection. PPO protein levels may, without limitation, be measured by enzyme activity assays, ELISA and Western blot analysis. PPO expression may be used as a predictor of reduced fruit browning. PPO enzymatic activity may be assessed biochemically or functionally.

For example, PPO activity may be measured biochemically by methods known in the art including, but not limited to, the detection of products formed by the enzyme in the presence of any number of heterologous substrates, for example, 4-methyl catechol. PPO activity may also be measured functionally, for example, by assessing its effects on fruit browning.

A genetically modified fruit-producing plant of the present invention may result in the reduction of total PPO activity in said plant or its seed, seedling, progeny thereof, or produced fruit thereof, by at least 57%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, or of at least 89%, relative to a wild type plant, seed, seedling, progeny thereof, or produced fruit thereof.

The skilled person would appreciate that the reduction in PPO activity may vary depending on a number of factors including, but not limited to, the source of the PPO, the developmental stage of a plant or plant material, the method of cultivation, the harvesting conditions, the experimental conditions and variations thereof.

In one embodiment, the PPO specific activity of tissue culture leaf material produced from the genetically modified fruit-producing plant of the present invention may be reduced by at least 250, at least 500, at least 750, at least 1000, at least 1250, at least 1500, at least 1750, at least 2000 or at least 2250 U/mg of protein as determined with the PPO specific activity assay of Broothaerts et al (2000), or a modification thereof, adapted for use in microtitre plate format; wherein the PPO specific activity of said wildtype of said plant averages 2630 U/mg of protein.

In another embodiment, the PPO specific activity of immature fruit material produced from the genetically modified fruit-producing plant of the present invention may be reduced by at least 10000, at least 15000, at least 20000, at least 25000, at least 30000, at least 35000, at least 40000, at least 45000, at least 50000, at least 55000, at least 60000, at least 65000, or at least 70000 U/mg of protein as determined with the PPO specific activity assay of Broothaerts et al (2000), or a modification thereof, adapted for use in microtitre plate format; wherein the PPO specific activity of said wildtype of said plant averages 75160 U/mg of protein.

A genetically modified fruit-producing plant of the present invention may result in the reduction of total PPO expression in said plant or its seed, seedling, progeny thereof, or produced fruit thereof, by at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, relative to a wild type plant, seed, seedling, progeny thereof, or produced fruit thereof.

Fruit Browning

As used herein, “reduced-browning” (or similar terminology) means that when a fruit, such as an apple, is bruised, sliced, juiced or processed in a manner where cell wall destruction takes place, browning will be detectably less than in a control. Any reduction in fruit browning (such as a reduction in browning visible to the naked eye relative to a control) may be advantageous. In one embodiment, the rate of browning of a fruit produced from a fruit-producing plant of the invention is reduced, relative to a control fruit. In another embodiment, the total quantity or degree of browning of a fruit produced from a fruit-producing plant of the invention is reduced, relative to a control fruit.

For example, an individual eating an apple would likely find it advantageous if the apple browned more slowly than a regular apple as in the case of Ambrosia. If that same apple were cut to be packed into a lunch or served on a fruit plate, then the apple would likely have to brown very little over an extended period of time in order to be acceptable.

Also, a consumer may indicate that some browning is acceptable, as this is the normal expectation. However, that same consumer would not purchase an apple from the store if it had any form of bruise at the time of purchase. So that consumer had already made considerable selection against apples that bruise.

Thus, industry would prefer an apple that did not suffer from bruising (which results in shrinkage) or browning (which results in lower consumer satisfaction). In other words, an apple that does not show visible evidence of browning from mechanical damage such as bruising or slicing, may have commercial advantages.

Any detectable level of reduced browning that is detectable to the naked eye may constitute a reduction in browning. Beyond this, reduced browning may be detected by a device, such as a chromameter, even if not visible to the human eye. Reduction in fruit browning may be determined in many ways, including, but not limited to, the difference in luminosity of a fruit tissue that is bruised in comparison to adjacent, unbruised tissue of the same fruit.

Fruit browning may be determined by known methods including, but not limited to, spectroscopy (e.g. light absorption, laser-induced fluorescence spectroscopy, time-delayed integration spectroscopy, large aperture spectrometer); colorimetry (e.g. tristimulus, “spekol” spectrocolorimeter); and visual inspection/scoring. These approaches allow the detection of, among other parameters, changes in luminosity and color of bruised transgenic fruit in comparison to bruised control fruit.

In one embodiment, a bruising apparatus may be used to deliver a controlled bruise to fruit with minimal destruction to the tissue. The particular specifications of such an apparatus are not critical. What is important is that the apparatus permits fruit to be bruised in a consistent manner so that the fruit may be used in controlled scientific studies.

In one embodiment, browning may be measured by the change in luminosity (ΔL) or total change in color (ΔE) assays described herein.

Luminosity may be measured and expressed in terms of any number of models including, but not limited to, the Hunter Lab color space or a related implementation thereof (see, for example, Hunter (1948a and 1948b)). In the Hunter Lab color space, L is a correlate of lightness which ranges from 0-100, where 100 is white and 0 is black; a and b are termed opponent color axes; a represents roughly Redness (positive) and Greenness (Negative), b is positive for yellow colors and negative for blue colors.

ΔL represents the change in luminosity between the unbruised apple (trt1) and the bruised apple flesh (trt2). A decrease in luminosity of 2.0 units (ΔL=−2.0) or greater, using the Hunter color space model, is generally visible to the eye.

ΔE represents the change in total color between the unbruised apple and the bruised apple flesh. ΔE is calculated from the formulae: ΔL=L _(trt2) −L _(trt1) Δa=L _(trt2) −L _(trt1) Δb=L _(trt2) −L _(trt1) ΔE=√{square root over ((ΔΔL ²+(ΔΔa ²+(ΔΔb²)}

A threshold for determining that a fruit has a reduced-browning phenotype is obtained when the ΔL (i.e. difference in luminosity between a bruised area of a fruit and adjacent unbruised tissue) is less than about 0.5, less than about 1.0, less than about 1.5, less than about 2.0, less than about 2.5, less than about 3.0, less than about 3.5, or less than about 4.0, using the Hunter color space model.

In one embodiment, in Golden Delicious apples, a decrease in luminosity of about 2.0 (ΔL=−2.0) is generally visible to the eye. For other apple species and/or varieties the ΔL value that represents a visible bruise may vary slightly from this depending on a number of factors, including, but not limited to, the natural flesh color of the apple (a and b color opponents). In another embodiment, an apple may be detectably low-browning to the naked eye if the decrease in luminosity is between about 2.1 to 3.5 units.

Fruit browning may also be considered with respect to a wildtype or control fruit and a test fruit produced from a reduced-browning plant of the invention. In an example, a control fruit and a test fruit produced from a reduced-browning plant of the invention are bruised in the same, or substantially the same way. Subsequently, the change in luminosity ΔL of each fruit is measured by detecting the luminosity at the site of bruising in comparison to the luminosity at an adjacent, unbruised site, resulting in a control ΔL and a test sample ΔL. The test sample fruit may be considered reduced-browning if the test sample ΔL is at least 1%, at least 2%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, at least 8%, at least 9%, at least 10%, at least 11%, at least 12%, at least 13%, at least 14%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% less than the control ΔL.

The skilled person would appreciate that browning may vary depending on a number of factors including, but not limited to, the manner and ambient conditions in which a fruit or plant material is bruised. For example, a fruit bruised at 2° C. may show different browning characteristics from a fruit bruised at 18° C., as detected by the eye or by an instrument, such as a chromameter, or like devices.

The skilled person would also appreciate that fruit and/or other plant material may be bruised in any number of ways. In one embodiment, fruit may be bruised according to the Controlled Bruising Procedure as follows.

Controlled Bruising Procedure

Summary

A controlled bruise is delivered to fruit (e.g. apples) in a controlled manner with minimal destruction of tissue using an impact device as described herein. Bruise response is reported as Change in Luminosity (ΔL), or Total Change in Colour (ΔL) between the bruised and non-bruised tissue as measured using a Minolta Chroma Meter.

Equipment and Materials

The Impact Device

The Impact Device comprises the Impact Device itself, plus a shallow container of glass beads into which the fruit is set, prior to being bruised. The Impact Device comprises a wooden block with a rounded impact surface that can be dropped from a consistent and adjustable height. A bruise, as delivered by the Impact Device could, alternatively, be produced by dropping a marble or steel ball down a tube, of a specific length, which is placed on the surface of the fruit. A shallow dish full of glass beads, into which the fruit is placed, provides a cushion to prevent damage to the underside of the fruit during impact to the top side of the fruit.

The fruit is ideally bruised with minimum tissue damage. Excessive impact can damage tissue and produce a Change in Luminosity that is unrelated to the bruising.

Colour Meter

A colour meter, e.g. a Minolta colour meter, is used to measure bruising. The meter is calibrated according to manufacturer's instructions against a white background.

Procedure

Fruit is removed from storage and allowed to come to room temperature for 2 hours. Positions of the bruises are marked with a felt pen on the fruit skin. Each fruit is bruised 5× and allowed to sit at room temperature for 3 hours for the bruise to form. The fruit are peeled over the bruise areas without removing the pen marking or cutting deeply into the flesh of the fruit. Each peeled area is measured on the non-bruised area adjacent to the bruise (trt 1) and directly on the bruised area (trt 2). Bruising, or Change in Luminosity (ΔL) is calculated as: ΔL=trt2−trt1.

For further details concerning measurement of fruit bruising additional information, see e.g. Rojas-Gra{acute over (ú)} M. A. et al. (2006); Gnanasekharan V. et al. (1992); and McGuire R. G. (1992).

Uses of Fruit with Reduced Browning

Fruits, vegetables and/or plants of the present invention may have commercial advantages. For example, reduced-browning produce that are easily bruised would retain the appearance of undamaged fruit, thus retaining its commercial value. In other examples, the juice of fruits and vegetables; or cut fruits and vegetables of the present invention would not require treatment with chemicals or other products to prevent browning, thus retaining the flavour and wholesomeness of the product. Accordingly, the genetically modified fruit-producing plants of the present invention may have commercial advantages in the food industry, for example, grocery, baked goods, beverages and snack industries; in advertisements and the advertising industry; in television programs (e.g. cooking shows); and in any business in which fruits and vegetables susceptible to browning are featured or displayed.

The invention is further illustrated by the following, non-limiting examples.

EXAMPLES 1. Reduction of a Single PPO Gene in Apples

In an illustrative example, the inventors have used a 250 bp fragment of AP14 between the Cu binding sites (or that includes one of the Cu binding sites) in the antisense orientation under control of the CAMV35S promoter (P_(CAMV35S)) and Nopaline Synthase terminator (T_(NOS)), which fragment was cloned into the binary vector pBINPLUS (van Engelen et al. 1995) to create the vector GEN-01. It was believed that the homology between all PPO sequences was sufficient that targeting any one of them would result in sufficient reduction of total PPO expression.

AP14 is highly homologous to GPO3 at the 5′ end. Over the region of AP14 cloned and sequenced, AP14 is 90% identical to GPO3 at the nucleotide sequence level, and 81% identical to GPO3 at the amino acid level (FIG. 1). However, there is a change in the AP14 coding sequence that generates a translational stop codon in the AP14 sequence, and approximately 43 base pair (bp) downstream of this stop codon, there is a 7 base pair deletion in AP14, relative to GPO3, that would create a frameshift in the coding region. Therefore, it was thought that AP14 is likely a pseudogene copy of GPO3.

The inventors have also used a PPO suppression transgene in which approximately 800 bp of APO5 was placed in the antisense orientation under control of the CAMV35S promoter (P_(CAMV35S)) and Nopaline Synthase terminator (T_(NOS)) and cloned into the binary vector pBINPLUS to create the vector GEN-02, which was based on information that showed that APO5 was the predominant species present in immature fruit. Additionally, Haruta et al. (1998) had reported reduced browning in leaf tissue caused by antisense construct that carried PPO3, a homolog of APO5.

The inventors produced over 200 GEN-01 and 400 GEN-02 genetically modified lines and have identified no lines with significantly reduced PPO Activity or reduced browning phenotype.

2. Cloning and Sequencing of Apple PPO Genes

It was recognized that apple PPO activity was encoded by more than one PPO gene sequence although the sequences had not all been cloned and the family had not been clearly established.

Robinson (1993) identified pSR7 and pSR8 (later to be identified as PPO2), HortResearch found PPO2, GPO3, APO5 and pSR7 in their apple EST library (personal communication). Boss et al. (1995) identified APO5 and Haruta et al. (1998) identified PPO3 and PPO7 (APO5 homologs). Kim et al. (2001) identified PPO2. Boss also identified GPO3 (unpublished).

The inventors used PCR using degenerate PPO primers to screen apple for novel PPO gene sequences. Using this approach, GPO3, APO5 and AP14 were identified in genomic DNA; GPO3, APO5 and PPO2 (PPOJ) were identified in apple fruit and apple leaf cDNA; and GPO3 and pSR7 were identified in an immature apple fruit cDNA library (Eugentech).

TABLE I Genbank PPO Gene Sequence Source Accession Sequence APO5 Boss et al. 1995 L29450 Complete HR_PPO2 PPO3 Haruta et al. 1998 D87669 Complete PPO7 Haruta et al. 1998 D87670 PPO2 Kim et al. 2001 AF380300 Complete HR_PPO3 PPOJ Okanagan Specialty Fruits pSR8 Robinson (WO9302195) A27663 Partial GPO3 Boss (personal Partial communication) Complete HR_PPO1 AP14 Okanagan Specialty Fruits pSR7 Robinson (WO9302195) A27661 Partial HR_PPO4/5, HR_PPO8 Complete Okanagan Specialty Fruits APO3 (5′ APO3 Boss (personal Partial sequence.) communication) APO9 (5′ APO9 Boss (personal Partial sequence.) communication) APO3 (3′ APO3 Boss (personal Partial sequence.) communication) APO9 (3′ APO3 Boss (personal Partial sequence.) communication)

The apple PPO gene sequences thus obtained (FIG. 2) were aligned using Clustal W™ (FIG. 3) and were sorted into four groups using ContigExpress™ (Vector NTI™ Suite 9.0.0, Invitrogen) and the groups were named for the PPO sequence type.

Referring to FIG. 3, sequences APO95 and APO93 are 5′ and 3′ non-overlapping fragments of the same clone. Similarly, sequences APO35 and APO33 are 5′ and 3′ non-overlapping fragments of the same clone. The 3′ sequences are identical while the 5′ ends do not overlap. Based on the 3′ sequence, it is assumed that the APO9 and APO3 clones are the same. Where APO9 overlaps the GPO3 sequence, they are 87% identical. Where APO3 overlaps the GPO3 sequence, they are 94% identical.

According to the alignment results obtained, it is expected that certain sequences are likely the same gene, or are likely equivalent from an antisense point of view (see Table II).

TABLE II Antisense Group Targets PPO2 PPO2, PPOJ, pSR8 GPO3 GPO3, AP14, APO9, APO3 APO5 APO5, PPO3, PPO7 pSR7 pSR7

Accordingly, the four apple PPO gene groups are: APO5 (includes APO5, PPO3 and PPO7), GPO3 (includes GALPO3, APO3, APO9 and AP14), PPO2 (includes PPO2 and pSR8) and pSR7 (FIG. 4). Alignment of the four PPO genes (PPO2, GPO3, APO5 and pSR7) with Clustal W showed an overall homology/identity between these sequences of 61 to 75% (see Table III).

TABLE III Homology/identity of the four apple PPO genes PPO2 GPO3 APO5 pSR7 PPO2 100 61 63 66 GPO3 100 75 62 APO5 100 65 pSR7 100 RNA Extraction from Tissue

Total RNA for RT-PCR was isolated from various apple tissues using a novel cellulose method (Weirsma 2001, submitted). Briefly, 2 g of frozen ground tissue was weighed out into a frozen 50 ml centrifuge tube. Extraction buffer (10 ml) preheated to 65° C. was added and the mixture was incubated for 1 minute at 65° C. to melt the tissue. The mixture was extracted one time with 5 ml of chloroform:isoamyl alcohol (24:1) with shaking for 10 minutes at room temperature. After centrifugation, the aqueous layer was filtered through miracloth into a fresh 50 ml centrifuge tube. Ethanol was added to the aqueous layer to a final concentration of 30% and 0.5 g CC41 (Whatman) cellulose powder was added. The mixture was shaken for 45 minutes on ice. The mixture was applied to a BioRad Econo-column and washed with approximately 250 ml of STE (30% STE: 0.1 M NaCl, 0.05 M Tris and 30% ethanol). After washing, the column was allowed to go dry and residual STE was purged from the column using a 60 ml syringe. ssRNA was eluted from the column with three elutions of 2, 1, and 1 ml of sterile double distilled water using an air purge in between each elution. Eluates were collected into a cold 50 ml centrifuge tube. Nucleic acids were precipitated by addition of 1/10 volume of 3 M NaAc pH 5.2 and 2.5 volumes of 95% ethanol overnight at −20° C. After centrifugation, the pellet was washed with 70% ethanol and aspirated dry. The RNA was solubilized in 600 μl of sterile RNAse-free water and transferred to a microfuge tube.

The 50 ml centrifuge tube was rinsed with an additional 300 μl of water which was also transferred to the microfuge tube (total volume in microfuge tube=900 μl). The ssRNA was selectively precipitated by addition of ⅓ volume of 8 M LiCl (2M LiCl final) overnight at −20° C. After centrifugation to collect the ssRNA, the ssRNA was solubilized in 400 μl of water. RNA was re-precipitated by addition of 40 μl of 3 M NaAc pH 5.2 and 1 ml of 95% ethanol. The RNA pellet was washed with 70% ethanol, aspirated dry and solubilized in 50 μl of water. The RNA was quantified spectrophotometrically and a sample was run on 1% TAE agarose gel to check integrity of RNA. RNA was stored at −80° C. until used.

EB: 0.2 M glycine, 0.1 M Na₂HPO₄, 0.6 M NaCl, 2% SDS, 2% PVP-40 and 5% BME

Degenerate PPO-Specific PCR Primers

PPO-specific degenerate PCR primers were developed using CODEHOP. Briefly, a consensus PPO amino acid sequence was generated from an alignment of the known apple PPO sequences (L29450, D87669, D87670, GALPO3) plus the sequences for apricot (AF020786), sweet potato (AB038994), pokeweed (D45385), tobacco (Y12501), tomato (Z12838), potato (U22922) and grape (Z27411). The alignment was submitted to the BLOCKS multiple alignment processor for arrangement into a format that is accepted by CODEHOP. The BLOCKS output was submitted to CODEHOP for selection of degenerate primers, using the Malus domestica codon table for back translation. CODEHOP degenerate primers were selected that were within the Copper binding sites and had similar melting temperatures (Tms). Primer JCA1 (5′ TCT TCT TCC CNT TCC ACC GTT ACt ayy tnt ayt t 3′) [SEQ ID NO: 69] and primer JCB1 (5′ CCA GCG GAG TAA AAA TTC ccc atr tcy tc 3′) [SEQ ID NO: 70] were selected. The primer JCA1 was modified to reflect codon usage in apple using the known apple DNA sequences.

Reverse Transcription

Reverse transcription (RT) (first strand synthesis) was carried out using Superscript™ II reverse transcriptase according to the manufacturers' instructions (Invitrogen). Briefly, an RNA/primer mixture was made that contained: 1 μg of total RNA, 1 μl of 10 mM dNTP and 1 μl of 2 μM cDNA primer in a final volume of 10 μl. The mixture was incubate at 65° C. for 5 minutes and then placed on ice for at least 1 minute. To this, 9 μl of a first strand synthesis reaction mixture containing: 4 μl of 5×RT buffer, 2 μl of 50 mM MgCl₂, 2 μl of 0.1 M DTT and 1 μl of RNAse Out™ was added. The RT reaction mixture was mixed gently, collected by brief centrifugation and incubated at 42° C. for 2 minutes. To the reaction, 1 μl of Superscript II reverse transcriptase was added and the reaction was incubated at 42° C. for 50 minutes. The reaction was terminated at 70° C. for 15 minutes and cooled on ice. Finally, 1 μl of RNAse H was added and the reaction was incubated at 37° C. for 20 minutes. The cDNA was used directly for PCR.

Hot Start, Touchdown PCR, TA Cloning and Sequencing

Hot start, touchdown PCR was used to amplify chromosomal DNA and cDNA samples. For amplification of genomic DNA, the PCR reaction contained: 1×PCR buffer, 1.5 mM MgCl₂, 200 μM dNTP, 1 μM JCA1, 1 μM JCB1, 1.25 U AmpliTaq Gold™, and 100 ng Golden Delicious genomic DNA. For amplification of cDNA, the PCR reaction contained: 1×PCR buffer, 1.5 mM MgCl₂, 200 μM dNTP, 1 μM PPO upper, 1 μM PPO lower, 1.25 U AmpliTaq Gold, and 2 μl of cDNA. The reaction was overlaid with oil and cycled. After an initial hot start incubation for 9 minutes at 95° C., the PCR reaction was subjected to 1 cycle of 95° C., 1 min; 70° C., 1 min; and 72° C., 1 min. The initial annealing temperature was reduced in each successive cycle by 2° C. to 62° C. The PCR reaction was subjected to a total of 40 cycles. After cycling, the PCR reaction was subjected to a final extension for 10 minute at 72° C., and then held at 6° C.

PCR products were size fractionated on TAE-agarose. Amplification products were excised from the gel, gel cleaned and ligated into pGEM-T Easy™. The ligation reaction was electroporated into Electromax™ DH10b cells. Plasmids carrying inserted were isolated and the insert was sequenced using M13F and M13R primers (BigDye™, ABI).

3. Reduction of PPO Expression in Apples

In a pivotal review RNA interference, Sharp (2001) suggested that for a transgene to induce silencing of a related but not identical target gene, the two segments must share regions of “identical and uninterrupted sequences of significant length” in the order of 30-35 base pair at a minimum.

Since dsRNA is processed to 21-23 nucleotide segments, Sharp suggests that a single basepair mismatch between the siRNA and target RNA dramatically reduce gene targeting and silencing.

The inventors compared pair-wise, the four PPO apple sequences, within a sliding (conservative) 22 base-pair window for regions of 100% homology.

This pair-wise analysis demonstrated that GPO3 and APO5, having an overall sequence similarity of 75%, have several regions of identical and uninterrupted sequences of significant length as shown below.

TABLE IIIb Number of Regions of 22 bp Micro Homology GPO3 APO5 pSR7 PPO2 0 0 0 GPO3 25 1 APO5 0 Table IIIb shows that: there are no regions of 100% micro homology between PPO2 and GPO3, APO5 or pSR7; there are no regions of 100% micro homology between APO5 and pSR7; and that there are 25 regions of 100% micro homology of 22 bp between GPO3 and APO5. These are part of 3 larger regions of 100% homology. There is 1 region of 100% micro homology of 22 bp between GPO3 and pSR7.

4. Construction of PPO Suppression Transgene

In an illustrative example, the inventors used a PPO suppression transgene PGAS (PPO2, GPO3, APO5 and pSR7), cloned in the sense orientation in the pBINPLUS to create GEN-03, to suppress PPO mRNA expression of all four PPO isoenzymes.

The PGAS suppression transgene was constructed using standard molecular biology techniques (Sambrook et al. 1989). Briefly, approximately 0.45 kb individual PPO fragments (PPOJ, GPO3, APO5 and pSR7) (FIG. 5A) were amplified from genomic DNA (GPO3, APO5) or cDNA (PPOJ, pSR7) using degenerate PCR primers, where the fragments include the Copper A and Copper B binding sites. PPOJ and PPO2 are 90% identical and could be the same gene, if for some reason the sequencing was poor. A person skilled in the art would expect that, from a functional perspective, suppression of either gene (PPOJ or PPO2) would be reasonably expected to induce suppression of the other. The fragments were cloned individually and then combined into a single chimeric PPO suppression fragment (PGAS) (FIG. 6A). Since the PPO fragments used in the PGAS transgene were initially amplified using degenerate PCR primers, the 5′ and 3′ ends of the PPO fragments used in the PGAS transgene may not exactly match the sequence of the endogenous PPO gene (FIG. 7A to 7L).

These PPO gene fragments were in the “sense” orientation under the control of the double enhanced CAMV35S promoter (P_(CAMV35S)) and Nopaline Synthase terminator (T_(NOS)) to create the PPO suppression transgene (P_(CAMV35S):PGAS:T_(NOS)). Approximately 50 bp of the PPOJ sequence was lost during the construction of the vector.

The P_(CAMV35S):PGAS:T_(NOS) transgene was transferred into pBINPLUS to create the plant transformation vector GEN-03 (FIG. 8A). GEN-03 was transferred into Agrobacterium tumefaciens LBA4404 in preparation for plant transformation. The elements of the GEN-03 T-DNA region that are transferred to the plant are described in (FIG. 9A-9C).

In another illustrative example, the inventors constructed an alternate PPO suppression transgene (PGAS2) (FIGS. 6B and 8B) having a 1771 bp chimeric PPO Suppression sequence comprising 200 bp fragment of each of four apple PPO genes (PPO2, GPO3, APO5 and pSR7; FIG. 5B), followed at its 3′ terminal by an apple intron, followed at the intron's 3′ terminal by inverted 200 bp fragment of each of pSR7, APO5, GPO3 and PPO2 (the same fragments as those used in 5′ of the intron). The fragments of PPO2, GPO3, APO5 and pSR7 used in the PGAS2 transgene were aligned with their respective genes (FIG. 7M-7X). The elements of the PGAS2 transgene are described in (FIG. 9D-9F); which was used to create the OSF-02 transformation vector (FIG. 8B). RNA transcribed from this 1771 bp chimeric PPO Suppression sequence is expected to generate a dsRNA stem of 800 bp with an intron loop. The transgene was under the control of the BUL409s promoter and the Ubiquitin 3 terminator (Garbarino and Belknap 1994).

The BUL409s promoter is a polyubiquitin promoter from the wild potato Solanum bulbocastanum containing 5′ regulatory sequences (784 bp), the 5′ untranslated region (59 bp), an intron (535 bp) and an ubiquitin coding domain (228 bp) (Bill Belknap, USDA Albany).

The Ubiquitin 3 terminator is the polyubiquitin terminator from potato Solanum tuberosum (DQ320121) (Garbarino and Belknap 1994).

5. Apple Transformation

A. GEN-03

The GEN-03 vector (having the P_(CAMV35S):PGAS:T_(NOS) transgene) was transformed into apple varieties Golden Delicious (also abbreviated as “GD”), Granny Smith (also abbreviated as “GS”), Fuji (also abbreviated as “Fu”) and Gala (also abbreviated as “Ga”), using Agrobacterium tumefaciens LBA4404 transformation.

Briefly, leaves of 3-week-old apple tissue culture plantlets were excised and cut into segments perpendicular to the mid-rib. Leaf segments were inoculated with Agrobacterium tumefaciens LBA4404 carrying the recombinant vector GEN-03 at a density of 3×10⁸ cells/ml for 5 to 10 minutes. Leaf segments were blotted on filter paper to remove excess bacterial cells and placed onto co-cultivation medium with the adaxial surfaces in contact with the medium for 4 days (dark). Infected leaf segments were washed and placed onto regeneration medium containing 6 mg/ml kanamycin with the adaxial surfaces in contact for 4 weeks (2 weeks dark, 2 weeks light). Leaf segments were transferred to regeneration medium containing 50 mg/ml kanamycin (4 weeks). Transformed shoots were transferred to proliferation medium with 50 mg/ml kanamycin (4 weeks). Surviving shoots were transferred to proliferation medium.

The selection marker (NptII) confers resistance to the antibiotic kanamycin. Cells that received and integrated the selection marker obtained the ability to regenerate in the presence of kanamycin. Shoots that arose from callus after the transformation process typically arose from a single cell that integrated the selection marker. These shoots were presumed to be homogenous.

Each shoot represented a unique transformation event and was genetically distinct. Individual shoots were given unique EventID numbers to identify the distinct genetic event (see Table IV). All plant material (tissue culture plants, field trees, tissue samples and apples) that arose from these single genetics events retained the EventID number.

TABLE IV EventID and PlantID Numbers (GEN-03) Event A genetically distinct transformation event. EventID Event Identification Number. A unique identification number assigned to each Event. Example: 705 Plant A tree either self-rooted or grafted. PlantID Plant Identification Number. A unique identification number assigned to each Plant. Example: 705-0001 The first plant of event number 705 B. OSF-02

The OSF-02 vector (having the PBUL409s:PGAS2:TUBI3 transgene) was transformed into apple varieties Golden Delicious, Granny Smith, Fuji and Gala, using Agrobacterium tumefaciens LBA4404 transformation.

Briefly, leaves of specially prepared Leaf Expansion Culture plants were excised and wounded with non-traumatic forceps (manufacturer). Wounded leaves were inoculated with Agrobacterium tumefaciens LBA4404 carrying the recombinant vector OSF-02 for 5 minutes. Leaves are blotted on filter paper to remove excess bacterial cells and placed onto co-cultivation medium right side up for 3 days at 25° C. (dark). Infected leaves were washed. The tip and base were cut from each leaf and the remaining leaf sliced into three sections. Leaf segments were transferred to regeneration medium (without antibiotics or other selection agents) and set in the dark for 3 weeks. Plates containing the transformed leaf segments were transferred to the growth room (without light). After 1 week, the lights were turned on. Over the next 3 to 6 weeks, regenerating shoots were transferred to proliferation medium (Cornell University).

Genetically modified plants were identified by PCR.

Sterile techniques may be used as described herein.

Alternatively, using sterile techniques in a laminar flow hood, 3-4 fully expanded, young leaves (10-20 mg) in good condition were taken from shoot cultures that were 3-4 weeks old and placed into labelled 96-Deepwell plates. The plates were transferred to the −80° C. freezer and then freeze dried for 24 hours at <100 m Torr. Plant genomic DNA was isolated from leaf tissue on an automated DNA extraction system using the “Slipstream MES” protocol (HortResearch).

Genomic DNA was amplified in a PCR reaction using PCR primers that are specific for endogenous APO5, the PGAS or PGAS2 transgene, the nptII selection marker (GEN-03) or vector backbone (OSF-02) (TABLE V-b1 and V-b2).

The BUL409s promoter (OBI-04/1313-1706) is unique to the PGAS2 transgene.

The backbone nptII sequence (nptII Backbone set 1) is present in the backbone of the OSF-02 vector. This sequence is not normally present in apple and will only be present in transgenic Events where the LB was bypassed (subjected to read through) during T-DNA transfer (during transformation).

6. PCR Screening for Genetically Modified Lines

Genetically modified lines were identified by PCR. Briefly, using sterile techniques in the laminar flow hood, 3-4 fully expanded, young tissue culture leaves in good condition were taken from shoot cultures that were 3-4 weeks old and placed into labeled FastPrep™ tubes. Each tube was placed immediately into liquid nitrogen and each batch of tubes was then transferred to the −80° C. freezer for storage. Plant genomic DNA for PCR was isolated from leaf tissue using CTAB (Lodhi al. 1994). Genomic DNA was amplified in a PCR reaction using the following PCR primers that are specific for endogenous APO5, the PGAS transgene or the nptII selection marker of GEN-03 (TABLE V-a1 and V-a2). For PCR primers used to screen lines for OSF-02, see (TABLE V-b1 and V-b2).

TABLE V Table V-a1: PCR primers for GEN-03 Size Screen for Genotype Target Primers (bp) Presence of Correct APO5 Apo5 Forward/Apo5 Reverse 250 PCR Positive Control Positive (Set1) APO5 JA4/JA5 800 PCR Positive Control Positive (Set2) PGAS CAMV35s/GPO3-R 953 CAMV35S:PPO2 Junction Positive (Set1) PGAS GPO3-L/APO5-R 672 GPO3:APO5 Junction Positive (Set2) PGAS pSR7-F (A81)/NOSTERM 556 pSR7:NOSTERM Junction Positive (Set3) nptII nptII Forward/nptII  286 nptII Selection Marker Positive (set1) Reverse nptII nptII-F/nptII-R 483 nptII Selection Marker Positive (Set2) Table V-a2: Primer Sequences (GEN-03) Target Primer Name SEQ ID NO: Primer Sequence (5′ to 3′) AP05 (Set1) Apo5 Forward 71 GCGTTGATTGTGGTTTCCTT Apo5 Reverse 72 TCCCGTTCCACCGTTACTAC AP05 (Set2) JA4 73 GCC GTC GAC CGA CGA CGA CCC ACG JA5 74 GCC GTC GAC AGC TGA GCC CAA GGA ATG PGAS (Set1) CAMV35s 75 ACA ATC CCA CTA TCC TTC GC GP03-R 76 CCT GGA TCT GGT TCA GTG C PGAS (Set2) GP03-L 77 TTC GCT AAC CCG GAC TCT APO5-R 78 CGG GTT CCC AAA GAA CAA CTT A PGAS (Set3) pSR7-F (A81) 79 GCC AAG CTT TTC CTT TCC ACC GCA TGT NOSTERM 80 TAT GAT AAT CAT CGC AAG AC nptII (Set 1) nptII Forward 81 CCT GCT TGC CGA ATA TCA T nptII Reverse 82 GAA ATC TCG TGA TGG CAG GT nptII (Set 2) nptII-F 83 GAA CAA GAT GGA TTG CAC GCA G nptII-F 84 CTG ATG CTC TTC GTC CAG ATC A Table V-b1: PCR primers for OSF-02 Product Size Screen for Genotype Target Primers (bp) Presence of Correct APO5 Apo5 Forward/Apo5 250 PCR Positive Control Positive Reverse BUL409s SP/OBI-04/1313-1706/ 394 PGAS2 Transgene Positive ASP/OBI-04/1313-1707 nptII (Set 1) Left/Right 172 Vector Backbone Negative nptII (Set 2) Left/Right 228 Vector Backbone Negative Table V-b2: Primer Sequences (OSF-02) Target Primer Name SEQ ID NO: Primer Sequence (5′ to 3′) APO5 Apo5 Forward 85 GCG TTG ATT GTG GTT TCC TT Apo5 Reverse 86 TCC CGT TCC ACC GTT ACT AC BUL409s SP/OBI-04/1313-1706 87 AGG GAG TGT GAA AAG CCC TA ASP/OBI-04/1313-1707 88 GGG GAG TTT GAA GTC GAT GA nptII (Set 1) Left 89 GAA AGC TGC CTG TTC CAA AG Right 90 GAA AGA GCC TGA TGC ACT CC nptII (Set 2) Left 91 CGG CTC CGT CGA TAC TAT GT Right 92 GCA GCG GTA TTT TTC GAT CA

APO5 is normally present in the genomic DNA of apple. A control amplification of an 800 bp fragment from apple genomic DNA with APO5-specific primers (JA4/JA5) showed that the particular DNA sample was amplifiable and that all PCR reaction components were in working order. Accordingly, all apple genomic DNA samples were expected to yield an 800 bp PCR fragment when amplified with these primers and only genomic DNA samples from which the APO5 was amplified were analyzed further.

While APO5 is present in the genomic DNA of untransformed plants, the CAMV35s:PPO2 junction, (CAMV35s/GPO3-R), the GPO3:APO5 junction (GPO3-L/APO5-R), and the pSR7:NOSTERM junction (pSR7-F/NOSTERM) are unique to the PGAS Transgene.

nptII is not normally present in untransformed apple tissue. Amplification of a 483 bp fragment from apple genomic DNA with NptII-specific primers (NptII-f/NptII-R) was evidence that NptII was present and that the tissue was therefore genetically modified tissue.

7. PPO Activity and Gene Expression

Briefly, using sterile techniques in the laminar flow hood, 6-10 fully expanded, young tissue culture leaves (approximately 10 mg) in good condition were taken from shoot cultures that were 3-4 weeks old and placed into labeled FastPrep™ tubes. Each tube was placed immediately into liquid nitrogen and each batch of tubes was then transferred to the −80° C. freezer for storage. Crude PPO was extracted from frozen ground leaf tissue. Total soluble protein was measured using BCA. Total PPO Activity was measured using 4-methyl catechol as substrate. The procedure was a modification of the Polyphenol Oxidase activity assay of Broothaerts et al (2000) adapted to a microtitre plate format.

In tissue culture, a survey of 34 untransformed Golden Delicious control samples, taken throughout the year, but from young healthy culture grown in nearly identical conditions, gave a PPO Specific Activity average of 2613+/−1019, with Specific Activity values ranging from 774-5995 U/mg protein. The Experimental Error of the PPO Assay was approximately 5-10%. Events were subjected to PPO activity screening preferably two times and more preferably > two times at successive sub-culture points.

Total RNA was extracted using a small-scale modification of the cellulose-binding method of Fils-Lycaon et al. (1996). Two g of powdered apple tissue in a 50 ml polypropylene Oak Ridge tube were shaken at room temperature for 45 min with: 9.33 ml GPS buffer (0.2 M Glycine, 0.1 M sodium phosphate (dibasic), 0.6 M NaCl, pH 9.5); 1 ml 20% (w/v) SDS; 0.3 g polyvinylpyrrolidone (PVP-40); 0.75 ml 2-mercaptoethanol; and 4.5 ml buffer-saturated (pH 8) phenol. Two ml of chloroform:isoamyl alcohol (24:1) were added and mixed briefly before centrifuging for 20 min at 14 k×g and 2° C. in a JA17 rotor (Beckman). Ten ml of the aqueous upper layer were carefully removed and filtered through Miracloth™ and 95% ethanol was added to bring the final ethanol concentration to 30% (4.5 ml 95% ethanol added to 10 ml sample). A 0.5 g quantity of cellulose (Whatman CC 51) was added and the slurry was shaken for 45 min on ice to bind the RNA. The cellulose was pelleted by centrifugation for 2 min at room temperature at 800×g, the supernatant discarded and the pellet resuspended in 40 ml 30% STE (30% (v/v) ethanol, 0.1 M NaCl, 50 mM Tris-HCl (pH 8.0), 1 mM EDTA). This wash was repeated five times with the final pellet resuspended in 25 ml 30% STE and poured into a sterile 1.5×15 cm nylon fritted column. The cellulose in the column was washed with an additional 200 ml of 30% STE and the residual buffer was expelled with air. Total RNA was eluted with RNase-free water, then precipitated with ethanol/sodium acetate in the cold and washed with 70% ethanol. The RNA was resuspended in water, residual cellulose was removed by centrifugation and the RNA quantified with RiboGreen™ (Invitrogen, Carlsbad, Calif.). cDNA was synthesized following a DNase I digestion from 1 μg of total RNA using Superscript II (Invitrogen) and oligo(dT) by a modification of the protocol of Huang et al. (2000). EDTA was added to chelate Mg before denaturation of the DNase I and additional Mg was added with the RT buffer to bring the final unchelated concentration to 5 mM. The RNase H step was omitted. The cDNA was diluted 4-fold with water, EDTA added to give a slight excess (0.5 mM) over Mg and stored at −20 C until use.

Real time PCR analysis was conducted in: 1× AmpliTaq™ Gold Buffer II; 2.5 mM MgCl2; 200 μM each dNTP; 7.5% glycerol; 3.0% DMSO; 1/40,000 SYBRGreen II; 0.5 unit AmpliTaq Gold; 200 nM each primer; cDNA equivalent to 6.25 ng of starting total RNA; and 20 μl final volume.

Primers were designed using Primer 3 (Rozen and Skaletsky, 2000). Primers are given in Table VI.

TABLE VI PCR Primers (5' to 3') Size Target Primer One Primer Two (bp) PPO2 MaldoPP02-69 [SEQ ID NO: 93] MaldoPP02-71 [SEQ ID NO: 94] 177 GGGACTCGCTCGACACTAAA TCACCTCGACGCTGATTGTA PPO2 MaldoPP02-69 [SEQ ID NO: 95] MaldoPPO2-70 [SEQ ID NO: 96] 229 GGGACTCGCTCGACACTAAA TCGTCATGTGCCTTCTTCTG GPO3 MaldoGP03-64 [SEQ ID NO: 97] MaldoGPO3-65 [SEQ ID NO: 98] 163 GTGAATGACGTGGACGATGA CATCATCTTCAGCACCCAAA GPO3 MaldoGP03-66 [SEQ ID NO: 99] MaldoGP03-67 [SEQ ID NO: 100] CATCTTCAGCACCCAAATCC TGAATGACGTGGACGATGAG 159 APO5 MaldoAP05-60 [SEQ ID NO: 101] MaldoAP05-61 [SEQ ID NO: 102] 218 AGTTTGCCGGAAGCTTTGTA TGATGCCTGGGTTGACATAA APO5 MaldoAP05-60 [SEQ ID NO: 103] MaldoAP05-62 [SEQ ID NO: 104] 219 AGTTTGCCGGAAGCTTTGTA TTGATGCCTGGGTTGACATA pSR7 MaldopSR7-53 [SEQ ID NO: 105] MaldopSR7-54 [SEQ ID NO: 106] 183 TAGTGTTCCGTGGCTGTTCA TCCTCCTCGTCGATCTTCTC pSR7 MaldopSR7-53 [SEQ ID NO: 107] MaldopSR7-56 [SEQ ID NO: 108] 270 TAGTGTTCCGTGGCTGTTCA CTGAGCGACTCAGCATCATC

A Stratagene Mx3000P instrument was used with cycle conditions of: 10 min 95° C. initial denaturation/enzyme activation; 40 cycles of 30 s at 95° C., 45 s at 60° C., 30 s at 72° C.; and with detection at the end of the 60° C. step. Dissociation curves were routinely run to ensure that single products were produced. Baselines and thresholds were set manually. Relative expression was calculated using the method of Pfaffl (2001) with efficiencies determined by the slopes of calibration curves using dilutions of cDNA as template. Normalization was done using an average of the expression values for the genes for protein disulphide isomerase (MdPDI1) and polyubiquitin (MdUBI2) which showed low variation in expression among the fruit samples. Measurement variation was maximized by using two cDNA synthesis reactions for each tissue and using these in separate real time PCR runs for the duplicate values plotted. Relative copy numbers were plotted on a logarithmic scale.

8. Micrografting

Lines showing highly reduced total PPO activity in tissue culture were grafted onto M9 (Malling 9) rootstocks and advanced into field trials according to Lane et al. (2003).

9. Controlled Bruising of Apples

Mature fruits were harvested from control and genetically modified apple lines and returned to laboratory for analysis. Fruits were subjected to a series of tests to determine whether the expected reduced browning phenotype followed the marked reduction in total PPO gene expression and total PPO activity. The inventors measured gene expression by quantitative PCR, total PPO activity, and browning response to slicing, impact bruising and juicing.

A special bruising apparatus was designed at PARC Summerland to deliver a controlled bruise to the apple with minimal destruction to the tissue. Apples were bruised in a consistent manner using the improvised Impact Device. Bruise response was reported as Change in Luminosity (ΔL) or Total Change in Color (ΔL) between the bruised and non-bruised tissue as measured using a Minolta Chroma Meter.

10. Results

A total of 184 Events were determined to be genetically modified by PCR screening. Of these, a total of 175 kanamycin-resistant Events were subjected to PPO activity screening (Events 717, 720, 721, 723-727, 735-737, 850, 881, 883, 884, 887 and 888 were not tested).

In one experiment, twelve GEN-03 Events plus untransformed control Golden Delicious or Granny Smith were selected for field trials (FIG. 10A). Some of these showed reduced-browning potential and others were sent as controls. In another experiment, twenty additional GEN-03 Events were selected for field trials (FIG. 10B). In other experiments, 10 OSF-02 Events were selected for field trials (FIG. 10C). In another experiment, 18 OSF-2 Events were selected for field trials (FIG. 10D).

Thirty-two GEN-03 Events were micrografted onto rootstocks (Malling 9), grown in the greenhouse/screenhouse and transferred into the field. Plants were grown in the field under standard commercial tree fruit management conditions.

Control and genetically modified fruits were harvested from the field trial and assessed. It is known that PPO gene expression is the highest and PPO protein is produced in immature fruit. Therefore, gene expression and Total PPO activity were measured in immature fruit harvested in the spring.

Detailed data from eight Events is provided to illustrate the relationship between total PPO activity in tissue culture leaf material, gene expression and total PPO activity in immature fruit and the desired non-browning phenotype achieved in mature apple fruit (FIG. 11).

Based on the herein described relationship between low tissue culture total PPO activity and the reduced-browning fruit phenotype, the inventors reasonably predicted that 13 Golden Delicious Events (702, 703, 705, 707, 730, 743, 752, 792, 801, 831, 842, 845 and 846), 1 Granny Smith Event (784), and 1 Fuji Event (872) should produce a reduced-browning fruit phenotype. In fact, Event 792 showed a reduced total PPO activity of 66% (FIG. 14B) and a ΔL of −1.1 and −0.5 in two independent experiments (FIG. 14C).

Three Golden Delicious Events (705, 707 and 743) had been selected initially showing reduction in total PPO activity in tissue culture of approximately 80%-90% relative to a control fruit. In immature fruit tissue, these Events showed significant reduction in gene expression of all four PPO genes. The suppression was more complete closer to the ends of the transgene and especially toward the 3′ end (pSR7). Decreased gene expression in immature fruit tissue was reflected in marked reduction in total PPO Activity of approximately 75-92%. Apples harvested from these Events were of a reduced-browning phenotype (low change in luminosity). Images of the “Controlled Bruising” provided in FIG. 12 clearly show that the change in luminosity that was measured in the reduced-browning Events was barely visible to the eye.

Juice produced from Event 743 did not significantly brown (FIG. 13 top). Even when left overnight at room temperature, while the untransformed control was darkened considerably within 15 minutes. It was also observed that the wet bruising often associated with damaged apple flesh did not occur in the reduced-browning Events.

The results obtained for Golden Delicious apples were similarly obtained in Granny Smith apple (784) (FIG. 13 bottom).

Many other Events were sent to the field for evaluation and the detailed results are reported in FIGS. 14A-14C.

Recovery of a large number of Golden Delicious Events reflects the emphasis on this apple variety for proof of concept of the reduced-browning technology and the amount of Golden Delicious material pushed through the transformation procedure, and should not be construed as limiting the invention.

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All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. 

The invention claimed is:
 1. A genetically modified apple plant, the fruit of said apple plant having reduced browning potential relative to said wild type, wherein the reduced browning potential results from the reduction in expression of genes encoding at least four polyphenol oxidase (PPO) isoenzymes in said apple plant relative to said wild type; or a genetically modified cell, seed, seedling, part, tissue, fruit or progeny of said plant, wherein said apple plant comprises: a first heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 18, that encodes a first PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 19, which first heterologous nucleic acid molecule reduces expression of said first PPO isoenzyme; a second heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 20, that encodes a second PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 21, which second heterologous nucleic acid molecule reduces expression of said second PPO isoenzyme; a third heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 22, that encodes a third PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 23, which third heterologous nucleic acid molecule reduces expression of said third PPO isoenzyme; and a fourth heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 24, that encodes a fourth PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 25, which fourth heterologous nucleic acid molecule reduces expression of said fourth PPO isoenzyme; and wherein said genetically modified cell, seed, seedling, part, tissue, fruit or progeny of said plant comprises said first, second, third, and fourth heterologous nucleic acid molecule.
 2. The genetically modified apple plant according to claim 1, wherein said first, second, third and fourth heterologous nucleic acid molecules are present in a single genetic construct and operably linked to a promoter in sense and/or antisense orientation.
 3. The genetically modified apple plant according to claim 2, wherein the genetic construct encodes an mRNA capable of forming a stem-loop structure.
 4. The genetically modified apple plant according to claim 1, wherein browning of fruit produced from said apple plant is reduced by at least 50%, at least 60%, at least 70%, at least 80% or at least 90% relative to a wildtype of said plant.
 5. A method for producing a genetically modified apple plant, said apple plant having reduced browning potential in fruit relative to a wild type of said apple plant, said method comprising reducing the expression of genes encoding at least four PPO isoenzymes in said apple plant relative to said wild type, wherein said method comprises: (a) introducing into an apple plant: a first heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 18, that encodes a first PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 19, which first heterologous nucleic acid molecule reduces expression of said first PPO isoenzyme; a second heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 20, that encodes a second PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 21, which second heterologous nucleic acid molecule reduces expression of said second PPO isoenzyme; a third heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 22, that encodes a third PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 23, which third heterologous nucleic acid molecule reduces expression of said third PPO isoenzyme; and a fourth heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 24, that encodes a fourth PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 25, which fourth heterologous nucleic acid molecule reduces expression of said fourth PPO isoenzyme; and (b) selecting and an apple plant comprising said first, second, third, and fourth heterologous nucleic acid molecules.
 6. The method according to claim 5, wherein said first, second, third and fourth heterologous nucleic acid molecules are present in a single genetic construct, and operably linked to a promoter in sense and/or antisense orientation.
 7. The method according to claim 6, wherein the genetic construct encodes an mRNA capable of forming a stem-loop structure.
 8. The method according to claim 5, wherein browning of fruit produced from said apple plant is reduced by at least 50%, at least 60%, at least 70%, at least 80% or at least 90% relative to a wild type of said plant.
 9. A method for producing a genetically modified apple plant, said apple plant having reduced browning potential in fruit relative to a wild type of said apple plant, said method comprising reducing the expression of genes encoding at least four PPO isoenzymes in an apple plant cell relative to a wild type apple plant cell, wherein said method comprises: (a) introducing into an apple plant cell: a first heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 18, that encodes a first PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 19, which first heterologous nucleic acid molecule reduces expression of said first PPO isoenzyme; a second heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 20, that encodes a second PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 21, which second heterologous nucleic acid molecule reduces expression of said second PPO isoenzyme; a third heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 22, that encodes a third PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 23, which third heterologous nucleic acid molecule reduces expression of said third PPO isoenzyme; and a fourth heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 24, that encodes a fourth PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 25, which fourth heterologous nucleic acid molecule reduces expression of said fourth PPO isoenzyme; (b) selecting and an apple plant cell comprising said first, second, third, and fourth heterologous nucleic acid molecules; and (c) regenerating an apple plant therefrom.
 10. A nucleic acid construct comprising: a promoter; a first heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 18, that encodes a first PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 19, which first heterologous nucleic acid molecule reduces expression of said first PPO isoenzyme; a second heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 20, that encodes a second PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 21, which second heterologous nucleic acid molecule reduces expression of said second PPO isoenzyme; a third heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 22, that encodes a third PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 23, which third heterologous nucleic acid molecule reduces expression of said third PPO isoenzyme; and a fourth heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 24, that encodes a fourth PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 25, which fourth heterologous nucleic acid molecule reduces expression of said fourth PPO isoenzyme; wherein the first, second, third, and fourth nucleic acid molecules are operably linked to said promoter in sense orientation.
 11. A nucleic acid construct encoding an mRNA capable of forming a stem loop structure, the nucleic acid construct comprising, from 5′ to 3′: a promoter, a first set of nucleic acid sequences, a spacer, and a second set of nucleic acid sequences, said first set of nucleic acid sequences comprising: a first heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 18, that encodes a first PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 19, which first heterologous nucleic acid molecule reduces expression of said first PPO isoenzyme; a second heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 20, that encodes a second PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 21, which second heterologous nucleic acid molecule reduces expression of said second PPO isoenzyme; a third heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 22, that encodes a third PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 23, which third heterologous nucleic acid molecule reduces expression of said third PPO isoenzyme; and a fourth heterologous nucleic acid molecule comprising at least 100 contiguous nucleotides of a sequence possessing 100% sequence identity to SEQ ID NO: 24, that encodes a fourth PPO isoenzyme comprising the amino acid sequence set forth in SEQ ID NO: 25, which fourth heterologous nucleic acid molecule reduces expression of said fourth PPO isoenzyme; said second set of nucleic acid sequences comprising: said first, second, third, and fourth nucleic acid sequences operably linked to said promoter in antisense orientation; wherein, the first and second sets of nucleic acid molecules are separated by a spacer.
 12. A genetically modified apple plant cell transformed with the nucleic acid construct of claim
 10. 13. A genetically modified apple plant comprising the genetically modified apple plant cell of claim
 12. 14. A genetically modified apple plant cell transformed with the nucleic acid construct of claim
 11. 15. A genetically modified apple plant comprising the genetically modified apple plant cell of claim
 14. 